@kuna When Opera loads into RAM upon starting, it must access various user files from your profile folder at the location defined when Opera was installed; if those files don't exist or are seriously corrupted, Opera will attempt to write default versions of the affected files to the specified profile folder location. Under 'About Opera', you should find the path to where Opera thinks the profile folder is located for that user account.
The error message you've noted indicates that permission to access a file(s) within the specified profile folder is inadequate for that which the OS believes to be necessary for the named folder. There are a variety of reasons this might occur, though most such reasons should operate all the time rather than intermittently. The intermittency would suggest that one of the data files needing to be accessed by Opera on occasion is perhaps located in a folder either in a protected system area or under a different user account, with the error message only occurring when that particular file is being called upon (which would not be all the time, else the error would occur all the time). It might alternatively suggest a corruption of something somewhere, either in an OS file, the Opera installation, the user profile, or even a problem chip in the RAM.
From within the user account and with Opera off, see if you can manually access the profile folder that was specified within Opera, as well as the files beneath that folder. Looking at the profile folder and various of its files, right-click and check the properties of each under the security tab to see if the full-control privilege is checked (or at the least modify, read & execute, read, and write privileges). If nothing obviously wrong appears there, try renaming the profile folder to some other name. Then restart Opera. Opera should immediately create a new profile folder and populate it with default versions of the files; try browsing for a while with Opera using these files (although your original settings, customizations, bookmarks, etc should all be missing after the renaming, unless you recreate them anew). If this fixes the problems, you can try copying over certain of the original profile files from the renamed folder to the new folder to recover their particular data (with Opera always turned off during such copying operations). In that case, however, do realize that copying in an original file risks copying in the original problem as well if it was caused by corruption in that original file.
If none of this works, you may want to try deleting the new-created profile folder and renaming the renamed profile folder back to its original name and then uninstalling/reinstalling Opera for all users from within an admin-level acount to try to clear up any possible corruption or privilege issues related to the install itself. In that case, Opera should recover and reuse all your user data in your profile.
@burnout426 Thanks, it seems like it's the only way to so it, so far
I hope they'll actually support Flash until 2020, not just abandon it, because "no one cares about these 15% of users", like Chromium actually did