"Retain tabs from previous session" only retains what's still open when Opera is closed. Closed windows are not saved and restored. That's why you must use Menu -> Exit and not close any windows when using multiple windows if you want each window to be saved.

In the "Sessions" folder of the profile I've only these 4 files (why not more older sessions?):

Chromium only decides to keep one past copy of each I guess.

What can I do?

Are there any bytes I can edit in these 2 files and then Opera uses my important tabs of window 1?

Not sure.

Are there descriptions of the file formats anywhere?

See https://digitalinvestigation.wordpress.com/tag/snss/ for example. (Note the article is a little old and takes about version 1 of the SNSS file format. According the hex you posted, Chromium uses version 3 now. I don't know what the differences are between versions.)

Does the "Session Buddy" extension work with multiple workspaces?

No. It has no knowledge of workspaces.

Again this morning with version 85.0.4341.60 (!).

All tabs of window 1 are gone. Only the 2-3 tabs of window 2 will be shown although for 100 % I closed the 2nd window first, opened a new tab on 1st window "to have an update" and closed it, turned off PC.

In the "Sessions" folder of the profile I've only these 4 files (why not more older sessions?):

Session_13293928681899796
Session_13293951582247730
Tabs_13293868496103388
Tabs_13293951540479050

and the first Session + Tab file are timestamped with 4-5 minutes past midnight (when I turned off the PC).

The others are created this morning. I've closed Opera and deleted them. Started Opera - again - only tabs from window 2.

I've made a HEX view on the first Session file and see the tabs which I want. But after start, Opera shows me only those from window 2 which I've closed 100 % before window 1. Why?

Here the first bytes of the Sessions file. Where you can see "https://win2day.at/" which was the first tab of my a workspace and now I miss it when I start Opera.

00000010h: 00 00 00 00 00 00 00 00 0A 00 00 69 05 00 00 01 ; ...........i....
00000020h: 00 00 00 09 00 09 B6 00 00 00 00 00 00 00 25 00 ; ......¶.......%.
00000030h: FE 20 00 00 00 B6 00 00 00 10 00 00 00 77 6F 72 ; þ ...¶.......wor
00000040h: 6B 73 70 61 63 65 73 5F 73 74 61 74 65 01 00 00 ; kspaces_state...
00000050h: 00 30 00 00 00 09 00 08 B6 00 00 00 56 00 00 00 ; .0......¶...V...
00000060h: 09 00 00 B6 00 00 00 B8 00 00 00 11 00 15 B8 00 ; ...¶...¸......¸.
00000070h: 00 00 00 00 00 00 B4 81 9E A2 46 00 00 00 85 0C ; ......´ž¢F...….
00000080h: 06 80 0C 00 00 B8 00 00 00 04 00 00 00 17 00 00 ; .€...¸..........
00000090h: 00 68 74 74 70 73 3A 2F 2F 77 77 77 2E 77 69 6E ; .https://www.win
000000a0h: 32 64 61 79 2E 61 74 2F 00 3E 00 00 00 4C 00 6F ; 2day.at/.>...L.o
000000b0h: 00 74 00 74 00 6F 00 20 00 2D 00 20 00 4F 00 6E ; .t.t.o. .-. .O.n
000000c0h: 00 6C 00 69 00 6E 00 65 00 20 00 43 00 61 00 73 ; .l.i.n.e. .C.a.s
000000d0h: 00 69 00 6E 00 6F 00 20 00 2D 00 20 00 53 00 70 ; .i.n.o. .-. .S.p
000000e0h: 00 6F 00 72 00 74 00 77 00 65 00 74 00 74 00 65 ; .o.r.t.w.e.t.t.e
000000f0h: 00 6E 00 20 00 2D 00 20 00 50 00 6F 00 6B 00 65 ; .n. .-. .P.o.k.e
00000100h: 00 72 00 20 00 75 00 6E 00 64 00 20 00 6D 00 65 ; .r. .u.n.d. .m.e
00000110h: 00 68 00 72 00 20 00 7C 00 20 00 77 00 69 00 6E ; .h.r. .|. .w.i.n
00000120h: 00 32 00 64 00 61 00 79 00 44 0B 00 00 40 0B 00 ; .2.d.a.y.D...@..
00000130h: 00 1E 00 00 00 38 0B 00 00 18 00 00 00 00 00 00 ; .....8..........

And here the first bytes of the Tabs file with a tab "https://www.discogs.com/" (also missing now):

00000000h: 53 4E 53 53 03 00 00 00 11 00 04 18 01 00 00 00 ; SNSS............
00000010h: 00 00 00 99 7C 46 FC 96 3A 2F 00 65 0B 01 60 0B ; ...™|Fü–:/.e..`.
00000020h: 00 00 18 01 00 00 00 00 00 00 94 00 00 00 68 74 ; ..........”...ht
00000030h: 74 70 73 3A 2F 2F 77 77 77 2E 64 69 73 63 6F 67 ; tps://www.discog
00000040h: 73 2E 63 6F 6D 2F 72 65 6C 65 61 73 65 2F 35 38 ; s.com/release/58
00000050h: 38 35 39 33 39 2D 31 32 2D 49 6E 63 68 2D 44 61 ; 85939-12-Inch-Da
00000060h: 6E 63 65 2D 38 30 73 2D 47 72 6F 6F 76 65 2F 68 ; nce-80s-Groove/h
00000070h: 69 73 74 6F 72 79 3F 75 74 6D 5F 63 61 6D 70 61 ; istory?utm_campa
00000080h: 69 67 6E 3D 72 65 6C 65 61 73 65 2D 75 70 64 61 ; ign=release-upda
00000090h: 74 65 26 75 74 6D 5F 73 6F 75 72 63 65 3D 72 65 ; te&utm_source=re
000000a0h: 6C 61 74 69 6F 6E 73 68 69 70 26 75 74 6D 5F 6D ; lationship&utm_m
000000b0h: 65 64 69 75 6D 3D 65 6D 61 69 6C 23 6C 61 74 65 ; edium=email#late
000000c0h: 73 74 2C 00 00 00 48 00 69 00 73 00 74 00 6F 00 ; st,...H.i.s.t.o.
000000d0h: 72 00 79 00 20 00 2D 00 20 00 56 00 61 00 72 00 ; r.y. .-. .V.a.r.
000000e0h: 69 00 6F 00 75 00 73 00 20 00 2D 00 20 00 31 00 ; i.o.u.s. .-. .1.
000000f0h: 32 00 20 00 49 00 6E 00 63 00 68 00 20 00 44 00 ; 2. .I.n.c.h. .D.
00000100h: 61 00 6E 00 63 00 65 00 20 00 38 00 30 00 73 00 ; a.n.c.e. .8.0.s.
00000110h: 20 00 47 00 72 00 6F 00 6F 00 76 00 65 00 BC 08 ;  .G.r.o.o.v.e.¼.
00000120h: 00 00 B8 08 00 00 1E 00 00 00 B0 08 00 00 18 00 ; ..¸.......°.....
00000130h: 00 00 00 00 00 00 10 00 00 00 00 00 00 00 10 00 ; ................

What can I do?

Are there any bytes I can edit in these 2 files and then Opera uses my important tabs of window 1? Are there descriptions of the file formats anywhere?

I really had some temporary tabs open "to-do" (to read). It takes hours to find them in the history and I don't want to do that (I have a family with 2 kids).

Until this problem is solved it is clear I have to use another browser as 2nd window. What should I do?

Does the "Session Buddy" extension work with multiple workspaces? I've never used extensions since I use Opera 6. I was happy as it was (and is).

UPDATE:
When I also open the new created Session and Tab files in HEX view, I can see all the HTTP entries of the tabs I'm missing. They are there or here - but Opera doesn't open them.

Thanks a lot,
BR

But from where the info of monitor 2?

Not sure. If I get a chance to test with multiple monitors, I'll see what I can find.

Although I've tested it yesterday morning and it has worked. Close window on monitor 2, close on monitor 1. Start Opera again and last session of monitor 1 was there. But why not from yesterday to today?

Not sure.

Session is unique, no matter if you have one or ten windows opened.

I've closed the one open window on monitor 2. Then I watched a film on Netflix on monitor 1 (my main window) for an hour. Closed it. Turn on this morning. Opera tabs all gone. The session from monitor 2 with one tab opens.

Although I've tested it yesterday morning and it has worked. Close window on monitor 2, close on monitor 1. Start Opera again and last session of monitor 1 was there. But why not from yesterday to today?

@burnout426 I know I can save a workspace into a speed dial folder. But this I would have to do 6 times (workspaces) everytime I close Opera (beside delete the old ones)! There are always some tabs which are added (for to read) and deleted.

I've also looked into the sessions folder. There are only 2 sessions files (session_, tabs_) (one from yesterday and one today). And the one from yesterday is the time when I've closed Opera after watching the film on Netflix and went to bed. 2 session files are to less.

The best would be: Opera writes after every tab change (or after a few seconds when opening multiple tabs within a short time) the complete session (all open tabs in all workspaces) as text file: "workspace, URL" into a CSV file with time stamp in name. And this for e.g. 100 times (and checks from time to time if old files have to be deleted).

If an accident happens I can always find a session I want. A copy & paste of this CSV format (or multiple URL's only!) into the address field should open them in one go. (btw: This feature request I really miss since the first days of Opera! I often have a list of URL's which I want to open in multiple tabs in one step). These session + tab files are binary format, but found one open URL inside the session file from yesterday which I want.

Test: I've renamed the session+tab file from yesterday. Closed Opera. Deleted the current session+tab file, renamed the old files back. So only 2 files in folder (from yesterday before I went to bed). Started Opera. Opera opens again with session from monitor 2 with one tab. From where? I'm able to find URL's in this required session and tab file but it looks like Opera doesn't use it. But from where the info of monitor 2?

I've thought I use MS Edge as 2nd window but it's not possible because I open many links from email newsletters and the default browser opens of course.

Also note that in each workspace, in each window, you can right-click on a tab, goto "save" and choose "all in a speed dial folder" to save all tabs in the workspace (or all selected tabs). You can then name that speed dial folder. Then, later on, if needed, you can right-click on that speed dial folder and choose to "open all in tabs" or "open all in workspace".

If you don't like those folders on you start page, at the URL opera://bookmakrs, you can move them to a folder in "other bookmarks" for example and open the folders of tabs that way if needed.

As for restoring what you lost, you can use "Recently closed" in the History sidebar panel to get some of them back. They'll even reopen in the correct workspace and maybe even the right window. For others, you'd have to use the regular history entries and have the right workspace selected and be in the right window.

Besides that, goto the URL opera://about and take note of the "profile" path. Then, close Opera and goto the profile folder. In the "sessions" folder you'll see timestamped files for sessions and tabs. You can play around with them by deleting the new ones to see if Opera uses the old ones. If your old tabs are in those older files, you might get them back. Make a backup of your sessions folder first though so you can restore from the backup if things go funky.

Just yet this morning, started Opera - and what happens: Opera starts with the one tab of window 2 and the new tabs from window 1 are gone again. How can this happen?