Restart Windows if you haven't.
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.
