Since it's copying related, try running two instances of imaptest where one copies messages and another runs on the destination box:
./imaptest logout=0 copy=100 copybox=Trash ./imaptest box=Trash append=0
It took about 1.5 hours, but I got 1..running the above 2 tests at the same time.
Sep 10 22:06:41 imap dovecot: IMAP(scorpio): rename(/var/spool/mail/dovecot-control/indexes/s/sc/scorpio/.Trash/dovecot.index.cache.lock, /var/spool/mail/dovecot-control/indexes/s/sc/scorpio/.Trash/dovecot.index.cache) failed: No such file or directory Sep 10 22:06:41 imap dovecot: IMAP(scorpio): file_dotlock_replace() failed with index cache file /var/spool/mail/dovecot-control/indexes/s/sc/scorpio/.Trash/dovecot.index.cache: No such file or directory
Thats on an unpatched server.. default 1.1.2 code. It seems to be a very rare occurance.
Cor