When using apache with a threaded MPM, this module can crash under certain conditions. Crash is dependent on the way apache assigns work to workers/threads, and can be influenced eg by moving resources within the site file structure causing symptoms to be obscure and mysterious; we ran this in production for nearly a year before a site reorganisation provoked the bug. I'll add a patch RSN but this should be fixed upstream.
It seems to be sufficient to add a mutex around the call to check_valid_cookie() in mod_auth_cookie_sql2.c (and link with a thread-safe mysql client library of course). Or avoid the problem by using the prefork MPM if that works for you.