Then, in Windows Explorer (File Explorer), goto "View -> Show" and turn on "file extensions" and "hidden items".

Then, in both "C:\Users\yourusername\AppData\Roaming" and "C:\Users\yourusername\AppData\Local", rename the "Opera Software" folder to "Opera Software old".

Then, start Opera and see how things go.

If you still have the issue, in both of those folders, right-click on the new "Opera Software" folders, goto "properties" and click the read only box so that it's clear (all white) and click apply. Then, switch to the "Security" tab, make sure your username is in the user list and make sure it has full permissions (all permissions set to "allow" except for "special permissions"). Click apply if you had to change anything and then OK out.

Then, try things again.

If still no go, make sure the permissions are correct for the "Roaming" and "Local" folders too.

If still no go, make a test standalone installation to a folder named "Opera Test" on your desktop to see if it has the issue too or not. If it does, it might be something outside of Opera causing it. If you think that's the case, investigate what security software you have installed on your system like any VPNs, firewalls, anti-virus software and CCleaner. You might have to temporarily uninstall (not just disable) them to truly test if they're interfering with Opera.

You can also uninstall Opera again and after it's uninstalled, delete the actual install folder that remains. It'd be the "Opera" folder in "C:\Users\yourusername\AppData\Local\Programs" or "C:\Program Files" depending on whether you have Opera installed just for the current user or all users. Then, delete everything in "C:\Users\yourusername\AppData\Local\temp" before trying to reinstall Opera.

If you get everything working again and it starts happening as soon as you renable Opera Sync again, it could be due to syncing settings. Try avoiding that. You might even have to goto https://www.sync.opera.com/ and choose to reset your passphrase to wipe out all your sync data on the server and then start over with a fresh user data folder again.

If you can still access your Opera account, check whether your bookmarks are available through Opera Sync on another device where you're signed in. Also, avoid resetting Sync or deleting any more profile data until you've confirmed that your bookmarks are backed up. If you previously exported your bookmarks as an HTML file, you can import them again through the bookmarks manager.

One thing worth checking is whether you're signed in to the same Opera account you used before. If you are, but your bookmarks and passwords still aren't appearing, it may be a Sync issue rather than something that another reinstall will fix.

Hopefully, your bookmarks are still associated with your account or available in a backup. Have you checked whether your bookmarks appear on another synced device?

First, check that Sync is actually bringing your bookmarks back: open Settings, go to Synchronisation, and make sure you are signed in to the same account you used before and that Bookmarks is switched on. Give it a few minutes, as a large set of bookmarks does not always appear straight away.

If nothing appears, the hard part is that deleting all user data also deleted the local copy, so only what had already been synced can come back. If you still have the old profile folder anywhere, for example in a backup, look for the Bookmarks file inside it and keep a copy before you try anything else.

One other thing worth checking: if you were signed in to a different account, even by a small spelling difference, Sync would restore a different, empty set of data. It is worth double-checking the account name shown in Synchronisation before trying another reinstall.