The problem was with the local data file, the sqlite DB holding my passwords, that I moved over (along with everything else) from my old Mac.
I had to delete it to add passwords.
This is a rather bad catch-22. First of all, moving my entire folder over should allow me to sync everything up (yes, I understand you have an Opera sync "feature". No, I'm not ever going to upload my passwords to you. Why in the world would you even want the liability of having possession of people's online banking / credit card passwords?). Second, if you're not going to let that work, then you need to wipe / rename / delete the non-functional password file.
Instead, you're giving the user the worst all all worlds: they can't get their old passwords, and they can't add new ones