[Dovecot] Problems with zlib

[Andre]

Sometimes I see lines like these in my log:

20100906 11:34:32 imap(username at domain.com): Panic: file istream-zlib.c: line 173 (i_stream_zlib_read): assertion failed: (zstream->high_pos == 0)
20100906 11:34:32 imap(username at domain.com): Error: Raw backtrace: /usr/lib/dovecot/libdovecot.so.0 [0x7f328d2244a2] -> /usr/lib/dovecot/libdovecot.so.0 [0x7f328d22450a] -> /usr/lib/dovecot/libdovecot.so.0(i_error+0) [0x7f328d2248b3] -> /usr/lib/dovecot/modules/lib20_zlib_plugin.so [0x7f328be444fc] -> /usr/lib/dovecot/libdovecot.so.0(i_stream_read+0x6b) [0x7f328d22a10b] -> /usr/lib/dovecot/libdovecot.so.0(i_stream_read_data+0x4d) [0x7f328d22a3fd] -> /usr/lib/dovecot/libdovecot.so.0 [0x7f328d21476e] -> /usr/lib/dovecot/libdovecot.so.0 [0x7f328d214cbe] -> /usr/lib/dovecot/libdovecot.so.0(message_parser_parse_next_block+0x29) [0x7f328d2148c9] -> /usr/lib/dovecot/libdovecot.so.0(message_parser_parse_body+0x23) [0x7f328d214a43] -> /usr/lib/dovecot/libdovecot-storage.so.0 [0x7f328d4cb72f] -> /usr/lib/dovecot/libdovecot-storage.so.0 [0x7f328d4cb84a] -> /usr/lib/dovecot/libdovecot-storage.so.0(index_mail_get_special+0x3dc) [0x7f328d4cc99c] -> /usr/lib/dovecot/libdovecot-storage.so.0 [0x7f328d4dd86d] -> dovecot/imap [username at domain.com 192.168.4.253 UID] [0x41256a] -> dovecot/imap [username at domain.com 192.168.4.253 UID](imap_fetch_more+0x16c) [0x412b0c] -> dovecot/imap [username at domain.com 192.168.4.253 UID](cmd_fetch+0x3fa) [0x40acea] -> dovecot/imap [username at domain.com 192.168.4.253 UID] [0x41031c] -> dovecot/imap [username at domain.com 192.168.4.253 UID] [0x4103cd] -> dovecot/imap [username at domain.com 192.168.4.253 UID](client_handle_input+0x45) [0x410545] -> dovecot/imap [username at domain.com 192.168.4.253 UID](client_input+0x62) [0x410f72] -> /usr/lib/dovecot/libdovecot.so.0(io_loop_handler_run+0xcb) [0x7f328d22f3ab] -> /usr/lib/dovecot/libdovecot.so.0(io_loop_run+0x18) [0x7f328d22e4c8] -> /usr/lib/dovecot/libdovecot.so.0(master_service_run+0x13) [0x7f328d21dd63] -> dovecot/imap [username at domain.com 192.168.4.253 UID](main+0x281) [0x419201] -> /lib/libc.so.6(__libc_start_main+0xe6) [0x7f328caa81a6] -> dovecot/imap [username at domain.com 192.168.4.253 UID] [0x408bd9]

Here is the backtrace:

[New process 11474]
#0  0x00007f328cabbed5 in raise () from /lib/libc.so.6
(gdb) bt
#0  0x00007f328cabbed5 in raise () from /lib/libc.so.6
#1  0x00007f328cabd3f3 in abort () from /lib/libc.so.6
#2  0x00007f328d2244b5 in ?? () from /usr/lib/dovecot/libdovecot.so.0
#3  0x00007f328d22450a in ?? () from /usr/lib/dovecot/libdovecot.so.0
#4  0x00007f328d2248b3 in i_panic () from /usr/lib/dovecot/libdovecot.so.0
#5  0x00007f328be444fc in ?? ()
   from /usr/lib/dovecot/modules/lib20_zlib_plugin.so
#6  0x00007f328d22a10b in i_stream_read ()
   from /usr/lib/dovecot/libdovecot.so.0
#7  0x00007f328d22a3fd in i_stream_read_data ()
   from /usr/lib/dovecot/libdovecot.so.0
#8  0x00007f328d21476e in ?? () from /usr/lib/dovecot/libdovecot.so.0
#9  0x00007f328d214cbe in ?? () from /usr/lib/dovecot/libdovecot.so.0
#10 0x00007f328d2148c9 in message_parser_parse_next_block ()
   from /usr/lib/dovecot/libdovecot.so.0
#11 0x00007f328d214a43 in message_parser_parse_body ()
   from /usr/lib/dovecot/libdovecot.so.0
#12 0x00007f328d4cb72f in ?? () from /usr/lib/dovecot/libdovecot-storage.so.0
#13 0x00007f328d4cb84a in ?? () from /usr/lib/dovecot/libdovecot-storage.so.0
#14 0x00007f328d4cc99c in index_mail_get_special ()
   from /usr/lib/dovecot/libdovecot-storage.so.0
#15 0x00007f328d4dd86d in ?? () from /usr/lib/dovecot/libdovecot-storage.so.0
#16 0x000000000041256a in ?? ()
#17 0x0000000000412b0c in imap_fetch_more ()
#18 0x000000000040acea in cmd_fetch ()
#19 0x000000000041031c in ?? ()
#20 0x00000000004103cd in ?? ()
#21 0x0000000000410545 in client_handle_input ()
#22 0x0000000000410f72 in client_input ()
#23 0x00007f328d22f3ab in io_loop_handler_run ()
   from /usr/lib/dovecot/libdovecot.so.0
#24 0x00007f328d22e4c8 in io_loop_run () from /usr/lib/dovecot/libdovecot.so.0
#25 0x00007f328d21dd63 in master_service_run ()
   from /usr/lib/dovecot/libdovecot.so.0
#26 0x0000000000419201 in main ()

here is the relevant plugin section:

plugin {
  zlib_save = gz
  zlib_save_level = 6
}

What can I do to investigate deeper?

TIA
Andre