Ejabberd crashes 2 to 3 times a day with lot of MUCc lients

Hi,
we excessively use group chats (MUC) with ejabberd.

We have about 8 MUC Rooms on our server, with up to 150 Users in the largest rooms, posting between 10 and 20 messages a minute. And users are excessively joining and leaving rooms.
After some hours running fine, ejabberD crashes with the following log messages.

Our ejabberd Version:
2.1.5-3+squeeze1

erlang.log:

=CRASH REPORT==== 29-Feb-2012::10:13:27 ===
  crasher:
    initial call: mod_pubsub:init/1
    pid: <0.324.0>
    registered_name: []
    exception exit: {undef,
                        [{node_default,init,
                             ["pubsub.xxxxx.de",
                              "xxxxx.de",
                              [{access_createnode,pubsub_createnode},
                               {plugins,["default","pep"]}]]},
                         {lists,foreach,2},
                         {mod_pubsub,init_plugins,3},
                         {mod_pubsub,init,1},
                         {gen_server,init_it,6},
                         {proc_lib,init_p_do_apply,3}]}
      in function  gen_server:init_it/6
    ancestors: [ejabberd_sup,<0.41.0>]
    messages: []
    links: [<0.231.0>]
    dictionary: []
    trap_exit: false
    status: running
    heap_size: 377
    stack_size: 24
    reductions: 415
  neighbours:

ejabberd.log:

=ERROR REPORT==== 2012-02-29 10:13:27 ===
E(<0.41.0>:gen_mod:73) : {undef,
                             [{mod_ctlextra,start,
                                  ["xxxx.de",[]]},
                              {gen_mod,start_module,3},
                              {lists,foreach,2},
                              {ejabberd_app,start,2},
                              {application_master,start_it_old,4}]}

Would be thankful for help :)

Greets
Jan