> Ah, yes.  With M-g on the group it does a full update of the marks the
   > same way the old nnimap backend does ("UID FETCH FLAGS 1:*", I think),
   > but with a 'g' in the group buffer it uses QRESYNC together with the
   > information cached in .newsrc.eld.

   > Using QRESYNC failed on some servers in the early days of the rewritten
   > nnimap.

   > Using dovecot, I haven't seen any issues since the new nnimap was new
   > and still work in progress (August to November 2010, according to the
   > git log, 1b47313d04b1f29cc08ba329375cbb7aba0a6c33 seems to be the
   > earliest change).


   > Well, if it crops up again, try M-g.

ok
   > What IMAP server is this?


Gmail, for which unfortunately offlineimap and dovecot did not work well
(while they worked nicely with other, real imap servers)