Additional info for the bug:
After watching this for several days, I can say that the behaviour (chat crash with dozens of lines "connected, disconnected, etc.") described in my previous posting seems to happen exactly every 4 hours; the respective times are (all in UTC): 8:03, 12:03, 16:03, 20:03 and presumably (I did not stay up that long for watching the chat behaviour) 0:03 and 4:03 (give or take one minute - might be that my clock could be a minute off)
I have repetedly seen my guildmates complaining about chat crashes at the same times of the day as it happened to me, so it's definitely not just a problem on my side.