Edit: Even then you might guess wrong. ;^)
I thought it was related to the snap, it happened to be the first one, but the strange customization I had made to my system (replacing /root with a symlink) meant that, in fact, any snap I picked would have had the same problem.
In any case it proved to be neither a ‘snap’ nor a ‘snapd’ issue.
Probably there ought to be a ‘snap_user’ category where we post stuff like ‘What, exactly, is wrong with me?’
I’ll shortly be posting a report of a permission problem installing an snap, as a user. I suppose that there is no dedicated category for this because it is rare, but gee whiz, now I’m going to guess where to ask for help.
If ‘install-issues’ or anything else sounds better, by all means.