This exact thing happened to me today, just 10 minutes ago, and my Opera version is exactly the same.
I use the bookmarks bar, and the bookmarks are arranged in folders. Today I noticed that one of the folders had moved into another folder, making it a sub-folder. I tried to move the sub-folder back to its place in the bookmark bar, but it made Opera crash. I tried this several times unsuccessfully. Every time Opera crashed. The first times I ran Opera again, it returned to the session I was in with the tabs open, but then after a few crashes it stopped opening the tabs and I had to retrieve the pages I had read from History. On top of this Opera disabled the extensions, so I had to enable them again.