[Dovecot] Assertion fails (mail_cache_add) in dovecot 1.0 RC 2

[Jens]

The log looks like this:

dovecot: Jul 11 10:38:39 Error: IMAP(ni): file mail-cache-transaction.c:
line 703 (mail_cache_add): assertion failed:
 (file_field != (uint32_t)-1)
dovecot: Jul 11 10:38:39 Error: child 31316 (imap) killed with signal 6

What I did:
1. Start dovecot with a fresh account so that no folders exist.
2. Start Thunderbird and create a folder "Mails gespeichert auf nasdrive"
3. Put a message into the folder (with Outlook)
4. Edit this message with Thunderbird and Save it.
5. Thunderbird creates the "Draft" folder but the message never gets there.
6. The log file contains the message above!
7. Check if missing folder is source of issue: Edit the message again
with Thunderbird and Save it again.
8. The message appears properly in the Draft folder.

Additional Info:
# dovecot --version
1.0.rc2
# dovecot --build-options
Build options: ioloop=poll notify=dnotify ipv6 openssl
SQL drivers:
Passdb: checkpassword passwd passwd-file shadow
Userdb: checkpassword passwd prefetch passwd-file static

Regards, Jens