The last patch https://phabricator.kde.org/D4027 regarding this topic "solved" the problem by accidentally disabling the DBus connection in total. This patch should restore it and Connection now sends signals form main thread, which solves problems of failed login and error spam.
I had on my laptop the problem of logout not working. But this also happens without this patch, so it's another problem. Martin, could you test your DPMS with it which you described in https://bugs.kde.org/show_bug.cgi?id=374554?