Start up gajim with the startup function of KDE Plasma. 
Try to insert the password into the "unlock kde wallet" window. 
If you take just more then 10 seconds, the application does avoid wallet and
does offer to safe the password. It has of course no access to the real one. 

Put in the password, then unlock wallet. You need now to shutdown gajim, then
when you reopen, the problem is solved and gajim can connect. 

Correct behavior: when wallet integration is set, gajim should wait until the
last grace period of wallet is over before ignoring it. 
Actually in Leap 15.1 the behavior is correct: the application wait until the
wallet has been unlocked, only then it proceeds. You can also be in error with
the first attempt and the application still waits. This is correct.