So I did an svn pull a couple of days ago that apparently exposed some bad values in the config file.
I had a merry task of hunting it down, first I thought it was the imap server causing hiccups.
So I tried all sorts of crap including downgrading to an older copy of roundcube.

Finally I decided to diff -urN the conf files and just update everything.
And now I’m pleased to say cubemail works again for everybody.
(at least it should)

Sorry about the interruption of service but at least it didn’t take too long to fix imho.