I found a workaround for these two related bugs, i.e., 372564 and 372258. Apparently kwalletmanager works smoothly if launched without indicating any session, so you can either disable your session manager (via "control center" -> "kde components" -> "session manager") or just exclude kwalletmanager from session managing. I hope this helps while debian maintainers fix these two annoying bugs and provide us with the fixed packages. Regards G.