I found that there are many incorrect entries with the same incorrect passwords for different accounts.

Different accounts on the same domain? For example, if you have 5 different Gmail accounts and they all synced to the same password when you logged into one and clicked "update", you might be experiencing this issue that seems to be caused by Opera Sync and syncing of passwords.

To fixed a broken password file, goto the URL opera://about, take note of the "profile" folder, close Opera, and delete the "Login Auto" file in the profile folder. You'll of course lose all username/password data, but that will cause Opera to create a fresh file that isn't corrupted.

To prevent corruption again though, you might have to avoid syncing of passwords for now or avoid using the "update" dialog when logging in. For example, at the URL opera://settings/passwords, for any domain/site where you have multiple accounts, use the "add" button there to manually add all the username and password entries. Never use the "save" button for that domain/site. And, whenever you log in when a different account on that domain/site, never choose "update".