It appears it's a known Seamonkey bug.
One of the comments in the bugreport suggests fixing by hand the prefs in about:config
:
I think the problem is the leftover valid setting for the news server. Usually you can see the pref for a short time when you create an account manually. (...)
When you cancel the subscription (...) you end up with a mail.server.serverX.valid boolean pref set to false. This seems to confuse the heck out of the account wizard. The simple solution seems just to delete the pref. The not so simple solution would be to make sure that every mail.server.serverX has such a valid pref regardless of it's setting. This seems to work too.
On my part, I managed to solve the problem by deleting both news accounts. This made the New Account Wizard restore its normal behavior, so I was finally able to create a second mail account.