Am I being blind or are these two essentials really missing here?
Yes, I’m still a noob at ask.libreoffice.org, but I have been looking for them for 15 minutes by now, and not found anything helpful in Site Feedback or the FAQ, either.
Click on your avatar, then “preferences”, then “log out”. You can change your password on the Single Sign-On platform you used to sign up.
Thanks for your quick reply. I have been to my account preferences and checked all the available tabs, which are
Summary
Activity
Notifications
Messages
Badges
Preferences
but I don’t see any “log out” anywhere. What’s wrong?
Click your avatar (top right), then the human icon (rightmost tab, tooltip “preferences”), then “log out” (last entry).
Thank you, I’ve found it!
It’s weird though: I somehow didn’t get the menu there before, but instead was taken to the preferences page immediately.
The only thing I changed in the meantime was quickly enabling and, as it didn’t help, re-disabling javascript from piwik.documentfoundation.org.
Anyway, thanks again for your assistance. 
I’ve just read all the above about where/ how to log out; and where/ how to change my password, which I changed some months ago (so it should be possible again!). I even have a User Name. But no success as yet. I’m generally baffled by the website, as I have been since April 2022, and have avoided it. If ever there was a website that required a training course in order to use it, it is this one. (And I’m far from unappreciative of LibreOffice,
having made several donations.)