Couchbase memcached crashed - all nodes are unhealthy

We started to send about 150k request per minute in a cluster with 12 nodes.
The servers started to crash. I can't figure out the reason, but I suspect it is a problem with the memcached module

Here is a link for the cbcollect: http://docs.google.com/file/d/0B2nCT2mLzqgEWmdOM05Sc3k2NlU/edit

This is happening every minute:
Event Module Code Server Node Time
Service memcached exited on node 'ns_1@172.16.74.232' in 0.45s
supervisor_cushion001 ns_1@172.16.74.232 13:38:29 - Tue Jun 18, 2013
Port server memcached on node 'ns_1@172.16.74.232' exited with status 134. Restarting. Messages: Tue Jun 18 12:38:29.584248 GMT+4 3: Trying to connect to mccouch: "localhost:11213"
Tue Jun 18 12:38:29.584829 GMT+4 3: Connected to mccouch: "localhost:11213"
Tue Jun 18 12:38:29.594811 GMT+4 3: Extension support isn't implemented in this version of bucket_engine
Tue Jun 18 12:38:29.601103 GMT+4 3: Failed to load mutation log, falling back to key dump
memcached: src/stored-value.cc:154: mutation_type_t HashTable::insert(const Item&, bool, bool): Assertion `itm.getCas() != static_cast(-1)' failed. ns_port_server000 ns_1@172.16.74.232 13:38:29 - Tue Jun 18, 2013
Port server memcached on node 'ns_1@172.16.88.45' exited with status 134. Restarting. Messages: Tue Jun 18 12:38:29.622465 GMT+4 3: Trying to connect to mccouch: "localhost:11213"
Tue Jun 18 12:38:29.623028 GMT+4 3: Connected to mccouch: "localhost:11213"
Tue Jun 18 12:38:29.642986 GMT+4 3: Extension support isn't implemented in this version of bucket_engine
Tue Jun 18 12:38:29.653570 GMT+4 3: Failed to load mutation log, falling back to key dump
memcached: src/stored-value.cc:154: mutation_type_t HashTable::insert(const Item&, bool, bool): Assertion `itm.getCas() != static_cast(-1)' failed. ns_port_server000 ns_1@172.16.88.45 13:38:29 - Tue Jun 18, 2013
Service memcached exited on node 'ns_1@172.16.75.222' in 0.51s
supervisor_cushion001 ns_1@172.16.75.222 13:38:29 - Tue Jun 18, 2013