@biske I have downloaded and installed version 88.0.4412.74 : https://get.opera.com/ftp/pub/opera/desktop/88.0.4412.74/
If sync is activated, you will keep your bookmarks.
Check if bookmarks are properly synced in your Opera account.
When uninstalling your current Opera version (89...), or installing the 88 version, Do Not check "Delete my Opera user data".