Goto the URL opera://settings/passwords, click the 3 dots to the right of "saved passwords", and export your passwords to a csv file. Open the csv file in a text editor to confirm all your passwords are there.

If you need to save opened tabs, right-click on a tab in the tab bar, goto "save" and choose "all tabs to a speed dial folder". If you use Opera's workspaces feature, repeat for each workspace. You can right-click on the folder that's created on the start page and edit its name to specify what workspace the tabs are from.

Goto the URL opera://bookmarks, click the drop-down at the bottom left and choose to export your bookmarks as an HTML file.

Then, goto the URL opera://about, take note of the "profile" and "cache" paths, close down Opera and delete the profile and cache folders.

Then, start Opera, goto the URL opera://flags/#password-import, enable the import flag, restart Opera, goto the URL opera://settings/passwords, click the 3 dots to the right of "saved passwords" and import your password csv file that you created earlier.

Then, goto the URL opera://settings/importData, choose "Bookmarks HTML file" in the drop-down, point it to your bookmarks html file that you exported earlier, and import your bookmarks. Then, goto the URL opera://bookmarks and move any imported bookmarks and bookmark folders that are out of place to where they're supposed to go.

Then, if you used workspaces before, recreate them with the same names you had before.

Then, on the start page, to restore your opened tabs, right-click on the speed dial folder that contains bookmarks of your opened tabs, and choose "open all in tabs". For other workspaces (if any), switch to a workspace, and do "open all in tabs" for its corresponding speed dial folder containing the bookmarked opened tabs.

Then, install any extensions you had before and customize any settings you want.

If you used Opera Sync before, you need to log out of Opera Sync on all your other devices, start with new profiles in them, goto https://www.sync.opera.com/ (in another browser, just in case doing so in Opera logs you into sync) and reset your passphrase. Then, if you want, you can log into Opera Sync again and sync things. But, don't choose to sync settings, history, and opened tabs. Sync is a bit buggy for those things.

If you're using CCleaner, make sure it's not set to clear Opera's files. And, in Opera, make sure you don't set Opera to delete cookies on exit at the URL opera://settings/cookies.

For Youtube/Reddit bookmark icons, this issue and solution still applies.