I'm not 100% sure but I think this the same as bug #520310 ...
I observed that when I created a new folder named 'blabla' through IMAP it actually got created in my homedir as '~/Mail/(null)blabla'. So, as a workaround, I symlinked my mail-folders 'logcheck' and 'Sent' to '(null)logcheck' and '(null)Sent'. Works for the time being. I'm about to try the suggestion from #520310 as that seems to be the recommend way. .
