"Invalid mailbox name" with mutt + Dovecot

[Jessica Perry Hekman <jphekman@xxxxxxxxxxxx>]

Hi all. I have been happily using mutt on various Linux boxes over the 
years (currently Ubuntu gutsy, mutt 1.5.15). I connect via IMAP to my 
ISP, pair.com.

Two weeks ago pair replaced their IMAP server with a Dovecot 
installation. I do not know what their old server was. Now, when I 
attempt to access a mailbox other than my inbox with mutt (something 
like "=spam"), mutt tells me "invalid mailbox name." I can access INBOX 
just fine.

I have asked pair to look into the problem, but they haven't been able 
to find the answer, though I think they are still trying. It's getting 
really annoying to not have access to my various folders to store mail 
in. Lots of googling has failed to find people with similar problems.

Does anyone have any insights? Anything particular I should suggest that 
my ISP try? Any way I can change my mutt config to play better with 
Dovecot? Or is it time for a new ISP?

Relevant bits of .muttrc:

 set folder="imaps://misilay.pair.com/../mail"
 set spoolfile="imaps://misilay.pair.com"

My inbox on misilay.pair.com is in /var/mail/jphekman; my other mail 
folders are in ~/mail/.

Thanks,
Jessica