With the ‘git pull’, the rebuild worked like charm.
@Falco, I was indeed in the old master branch. Now I’m in the new main branch… I can’t really say where that happened, it’s quite possible that a repeat run of launcher did it under the hood (I do see some code in there that does the switch if you happen to be in master).
Ah well, I’ll call this upgrade “magic” and not care beyond that, 'cause now, my discourse instance is happily up and running.
Thanks for the (moral if nothing else) support, guys!