Yeah, for some strange reason people are having problems with their sessions files. It only effects a small number of users though. To keep it from happening you'll need to stop using 'Continue where I left off' in the options page or delete the sessions files every now and then. If you are really into the session feature I recommend using Session Buddy (on the chrome store) and the 'Download Chrome Extension' on the Opera store. Install the latter one first and then Session Buddy. Its a much more full featured session system.
In the good old Opera versions <= 12 it was possible with the described entry. And just after "dragging" an image some pixels it was opened in a new tab.
But dragging it onto the tab bar is a good workaround 😉
did you try resetting all flags?
opera://flags/
My bookmarks were gone because of tree-in-bookmarks flag, but Opera 27 still doesn't have that flag (I think)
maybe some other flag is messing with your bookmarks
Probably the installation options: all users, current user or standalone. Based on the location of the profile folder, standalone doesn't seem to be the case.
Do you have admin rights? Any program that may be clearing the files?