This error indicates that the server has run out of available slots for "inter-process communication". This can occur when a process crashes before cleaning up after itself.
ipcs command will list semaphores and shared memory slots in use. If there is a large number of entries in use by the apache user use
ipcrm to remove them.
Then, check your server's error log (possibly with
LogLevel debug set) to figure out why it's crashing, or see: http://httpd.apache.org/dev/debugging.html