Same problem here. I have five Jabber accounts, and only one has this behavior. I'd like to blame the server, but I see the prompt for the password even before a single network packet goes out.
So before anyone tries these steps, here's what did NOT work for me:
- Removing the account and adding it again
- Disabling Keychain iCloud sync
- Adding the account in System Preferences vs Messages
- Removing the entry from the keychain
The odd thing is that for all other accounts, if I go in the Accounts preferences in Messages.app, and then click on the Account Information tab, I see the username, and for password, I see "••••••••." Bit off the problem account, it's blank.
If I go offline, I can edit that field. But if I click on another account and then come back to this one, it's blank again. Huh? Clearly a bug. The only thing I see different about this account is that is has a ".io" domain. I thought that was the problem, so I set up another Cloudfire server on another .io domain, and it worked perfectly. I set up another on a .us domain - same problem - won't save the password. I can't find any pattern.