Eric,

have you tried going to server mode (^ from Group buffer) and then
selecting the appropriate server to see what groups are available from
that server?

The other thing to do is "G c" from the group buffer on that group and
see what "active" list is (at the end of the buffer you get).  Maybe
delete that entry and see what happens?

The latter is most likely to help at all as the problem is likely that
you have confused gnus by switching servers and the IDs of the messages
have changed.

I assume you are not using the gnus agent.

-- 
: Eric S Fraga, GnuPG: 0xFFFCF67D
: in Emacs 24.4.50.1 + Ma Gnus v0.12 + evil-git-7c4c180
: BBDB version 3.1.2 (2014-05-06 11:45:08 -0500)