Oh boy, I didn't expect #Matrix to be such a pain! So now I can successfully write messages to that one group channel from one PC, but when written from another PC they cannot be decrypted by others. I could start debugging this; but that's yet another layer of yak shaving :-(
Seriously, can we maybe go back to IRC? Trying to find out who can read which Matrix message in a channel that's about a wholly different topic is not even funny any more.