Didn't test, but this might work:

Make a test standalone installation and close it. In its install folder, go in to the profile/data folder and delete the Bookmarks and BookmarksExtras files. Then, in the "Sessions" folder there, delete all files, and put copies (so you still have the originals just in case) of your old session files there.

If you're lucky, when you start that test Opera, you'll see your opened tabs. Then, you can right-click on a tab in the tab bar, goto "save", and choose "all tabs to a speed dial folder".

Then, goto the URL opera://bookmarks and use the drop-down at the bottom left to export the bookmarks to an HTML file.

Then, in your regular Opera, goto the URL opera://settings/importData, select "Bookmarks HTML file" in the drop-down, and import the HTML file.

Then, at the URL opera://bookmarks, right-click on that imported speed dial folder and choose "open all in tabs".

I've seen a handful of forum topics about something similar, but nothing that solves my problem. Opera (GX) has been flashing red in the taskbar, and yes, even when my downloads list is empty and nothing is incomplete/errored. After a lot of testing, I feel that it is safe-ish to say I have found the problem.

When I alt-tab from a full screen game, and the alt tab page selector hovers on Opera GX, even if only for a second, Opera GX will flash red in the taskbar like there's no tomorrow. And yes, I have no failed downloads or errored downloads, and I'm on the latest build of Opera GX.

I am unsure if this is a problem specific to Opera GX, and not the Opera browser as a whole, but if anyone could help me that would be great. Maybe if somebody knows how to disable the taskbar flashing red for only this app, so I can keep other things flashing red too? Anyways, thanks for reading.

@keyuri I've just tested and it worked fine here.

@burnout426 - Well, I'm laughing and scratching my head. When I rebooted my machine today, I only had one shortcut to Opera, instead of the two I had yesterday. Grimacing, I clicked on the one shortcut, and it opened - it was the old profile, with all the extensions I use and Sync is running.
Just to check, I looked at History, and was I shocked! It's all there!! Not only the dates I had used on the new profile, but history going back to February!! So, it seems no history was actually missing at all - it just wouldn't show before.
Sooooo, I don't know what happened, but it looks like Opera fixed itself!

Thanks again for your help - you got the 'ball rolling'!

@doriemisur Run the installer again and click on options to see.