@misterarthur I had the same problem. I fixed the password problem by deleting the Opera entry using Keychain Access (I don't save many passwords). After that no more repeated password popups. However, whenever I close and re-open I was being asked to accept cookies.
For the moment I've uninstalled Opera and will use other browsers for a while. I'll probably give Opera another go in a few months, but clearly the quality on Mac is lacking.