OK, with the kind help of Naiomi and team at GS, it turned out that not having my existing BIMcloud active and running at the time I installed the second is probably what triggered a cascade of woe.
Essentially the new installation had by default likely tried to take over the same ports that the old installation was using at one point during the process (despite what I manually entered and later re-corrected in config settings), creating cross-linked or corrupted databases once both BIM clouds were active and trying to use them.
Once the damage is done, it seems that merely ensuring the correct ports are nominated in the respective BIMcloud server and manager configurators is not enough - you need to uninstall everything and start again (hope you had your projects backed up!), or work through an elaborate procedure that amounts to the same thing, though with not quite the same destruction of data
Moral of the story is to ensure all currently installed BIMclouds for your various ArchiCAD versions are running, before you try to install the next.
PAUL KING | https://www.prime.net.nz
ArchiCAD 8-28 | Twinmotion 2024
Windoze 11 PC | Intel Core i9 10900K | Nvidia Gforce RTX 3080 | 32 Gb DDR3 | 2x4K monitor extended desktop