Just to tie things together; here is a link to a discussion of the unwarranted banning problem, on Github:
I posted about my experience there, and other people have joined in.
Short version - if you get this message, clear your browsers cookies:
Sorry (username), you are banned from using this forum.
The ban is not set to expire
Yes - it really does say “Sock puppet”, it’s not a picture of one.
That may be enough. I also cleared a few other things while I was there:
"The actual step I took in Waterfox (same thing as Firefox, pretty much) was to go to History, then Clear Recent History. I had Cookies, Cache, Offline Website Data and Site Preferences all ticked, and no others, and the “Time range to clear” set to Today.
I then just refreshed a tab where the sock puppet ban message was showing, and it went away."
(that bit’s copied from a post I made in that Github thread).