500 error in MAMP after enabling module, yes I read the help docs

I'm having a terrible time with a local Drupal distribution of COD (Conference Organizing Distribution). It works fine and zippy, but I'll add modules to the sites/all/modules directory or enable them in the admin and it will go blank.

Yes, I've read the help docs and have increased my memory_limit to 128M and even 256M.
I'm running PHP 5.4.10 and xCache. I switched to eAccelerator and APC...and nothing.

The error is:

Fatal error: Uncaught exception 'PDOException' with message 'SQLSTATE[HY000]: General error: 2006 MySQL server has gone away' in /Users/path/to/mysite/includes/database/database.inc:2139 Stack trace: #0 /Users/path/to/mysite/includes/database/database.inc(2139): PDOStatement->execute(Array) #1 /Users/path/to/mysite/includes/database/database.inc(664): DatabaseStatementBase->execute(Array, Array) #2 /Users/path/to/mysite/includes/database/database.inc(2318): DatabaseConnection->query('SELECT expire, ...', Array, Array) #3 /Users/path/to/mysite/includes/lock.inc(167): db_query('SELECT expire, ...', Array) #4 /Users/path/to/mysite/includes/lock.inc(146): lock_may_be_available('theme_registry:...') #5 /Users/path/to/mysite/includes/theme.inc(447): lock_acquire('theme_registry:...') #6 /Users/path/to/mysite/includes/bootstrap.inc(442): ThemeRegistry->set(Array) #7 [internal function]: DrupalCacheArray->__destruct() #8 {main} thrown in /Users/path/to/mysite/includes/database/database.inc on line 2139