Thanks. Yes, the two users I keep trying to log in as are, indeed, allowed to access WordPress admin.
Also, I have to say that I’m floored! I’d finally managed to log into sso on Microsoft Edge (it’s redirecting in a loop on Brave, as mentioned above, and in Firefox it’s telling me that I should “Please log in from the portal”, something I’ve seen before on this forum but didn’t manage to pin down to specific settings or anything). But, after logging out, it wouldn’t let me log back in (sent me in the login loop, like Brave).
However, upon restarting Edge, it’s now allowing me to log in again 
All the while, though, the admin console (https://yunohost.domain.tld/yunohost/admin/#/) is usable in all browsers, thankfully.
I tried to look at all the logs I could think of to figure out what’s not going right. I did an ls -n and even a sudo find -printf "%TY-%Tm-%Td %TT %p\n" | sort -n on /var/log/ right after a redirected (looping) login attempt, to see which log files were the ones that had changed just then.
But nothing I read in either /var/log/nginx/yunohost.domain.tld-access.log, or in /var/log/debug or /var/log/daemon.log, etc. have helped me understand the issue… Please do direct me to which log file might record this behaviour, if any 