Can't select group on groups with unread messages

[Eric Abrahamsen <eric@ericabrahamsen.net>]

So I did something clever recently, switching the IMAP backends of two
of my nnimap servers, without telling Gnus I'd done it. Everything went
surprisingly well, except for one thing.

On one of my accounts (not the other), if the INBOX has unread messages,
I can't open it. If I hit return on it, it tells me "Can't select
group". If I open it with C-u or M-{digit}, it opens with the specified
number of messages. So far I'm not sure if it is showing me the newly
arrived messages or not; at any rate, when I open the group using C-u or
M-{digit}, none of the messages are marked unread, and when I quit out
of the group, the unread count remains unchanged. I still can't just
enter the group normally.

Does anyone know how I get out of this situation?

Thanks!
Eric