dbmail-imapd stops working, almost without a trace

Hi folks,

I've got a problem that dbmail-imapd stops working a few times a day. It 
started a few months ago about once in a while, but now occurs more and 
more often.

The only trace it leaves behind is in /var/log/dbmail.err:

dbmail-imapd: dm_string.c:160: p_string_erase: Assertion `pos + len <= 
S->used' failed.

I've also tried to set the debug flags, trying to isolate the problem 
from what's in the logs, but dbmail-imapd just dies, leaving this only 
trace in dbmail.err.

I was using dbmail 3.1.17 when the problem started, but I got annoyed 
when dbmail-imapd stopped working 3 times in one day, and upgraded to 
3.2.3, which did not help.

Nothing has been changed in the environment since more than a year, 
except Xen being updated from 4.4.1 to 4.5.0.

The mail server is using 2 VCPUs, the load is very low about 20 messages 
per day and just a couple of users. The load on the rest of the server 
is also very low. The MariaDB database resides on another virtual 
machine. The virtual machine has got plenty of free disk space and memory.

The environment has got the following properties:

Linux Slackware 14.1 64-bit
dom-U 64-bit kernel 3.13.1
glib-2.38.2
gmime-2.6.19
libevent-2.0.21-stable
libffi-3.2.1
libzdb-2.12
libsieve-2.3.1
perl-5.18
postfix-2.10.2
mysql client MariaDB-5.5.32


If somebody has got any idea what's up, I'd be grateful to get a fix, or 
any idea how to solve this annoying problem.

Best regards,

Peter