Apparently I can… I wonder why I couldn’t this morning.
Well, forum authentication depends on the availability of the API on www.openstreetmap.org. So if there is a problem with the API, then you cannot login to the forum.
I wonder what happened… My login on my laptop stopped working, and I checked it now and it seems like I had misspelled the password, which is strange because I think it was working before… Perhaps it used to discard the last characters or something like that…
I believe the username is now also case-sensitive.