Did I do something in AVG to cause this?

Not sure. If you haven't been messing with AVG's settings, I assume it's nothing you did.

I'll probably wait a week until reinstalling AVG to see if the problem's gone away. I'm at a loss, for AVG hasn't given me this problem before. Did I do something in AVG to cause this? I hope it isn't a permanent bug, since my family bought it and it's useful in protecting our computers.

I didn't like that, so I unrenamed "Opera Stable Old" but it asked if I wanted to combine it with the new 1. Initially I said yes but when it asked to do it for a sub folder I said no. I deleted the new Opera Stable and renamed the old Opera Stable. I opened Opera to find my bookmarks and such back. As a test, I deleted some old bookmarks, added a new 1, and edited another 1. When I reopened Opera to my surprise all the bookmark changes stuck! I don't know what I did right but I think it's fixed [knock on wood]. Thank you for the advice!

Shutting off AVG temporarily doesn’t help.

Test by uninstalling AVG instead to see if that makes a difference. You can reinstall it again when you're done testing.

but I couldn’t do that for Roaming’s version because it kept saying the folder or file was open in another program

Definitely need to figure out what has a lock on that folder. That's where all your settings and bookmarks etc. are stored.

Download Handle from https://learn.microsoft.com/en-us/sysinternals/downloads/handle to your downloads folder. Right-click the zip file and choose "extract all" to extract it to a folder named "Handle" in your downloads folder.

Then, hit Windows key + r to open a run dialog, type cmd and press enter to open a command prompt.

In the command prompt, type:

cd "%UserProfile%\Downloads\Handle"

and press enter.

Then, type:

handle64 "%AppData%\Opera Software\Opera Stable"

and press enter and give it a moment to spit out some logging. When done, hit ctrl + c to end the logging.

That should tell you the names and process IDs that are doing stuff to the "Opera Stable" folder. If there's nothing that has got a hold on the folder, it'll say "No matching handles found".

Then, you can open up the Windows Task Manager, find those processes (by name, and by ID if you enable the PID column) and end task on them. Then you can rename that folder.

If the process name is explorer.exe, you can select that in the task manager and click "restart task". That should then let go of the folder so you can rename it.

If you want, you can try https://lockhunter.com/ instead if that's easier.

I ran System Fille Checker in Command Prompt and it found nothing wrong. Deleting cookies didn’t solve the problem. I have noticed if you try to delete bookmarks or empty the bookmarks trash folder when you start Opera back up they’re right back where they originally were. I also deleted a few extensions and disabled all of them but that too gets reversed when Opera’s reopened. When I open a new window or private window when the current window with new bookmarks is still open, those new bookmarks are still present in those new windows.

Also see https://reddit.com/r/operabrowser/comments/1dwobrd/comment/lbyrhxz/?utm_source=reddit&utm_medium=web2x&context=3

Also see https://forums.opera.com/post/355044.