4.0.0 XCache Problem

[AL B]

After enabling xcache and uploading the constants.php file I get a server 500 error;

thrown in public_html/system/Data/Cache/Xcache.php on line 66
[09-Apr-2015 18:26:25 UTC] PHP Fatal error:  Uncaught exception 'ErrorException' with message 'xcache_set(): XCache var cache was not initialized properly. Check php log for actual reason' in public_html/system/Data/Cache/Xcache.php:66
Stack trace:
#0 [internal function]: IPS\IPS::errorHandler(2, 'xcache_set(): X...', '...', 66, Array)
#1 public_html/system/Data/Cache/Xcache.php(66): xcache_set('8f98b93f5b_sess...', '{"member_name":...')
#2 public_html/system/Data/AbstractData.php(90): IPS\Data\Cache\_Xcache->set('session_ada3748...', '{"member_name":...')
#3 public_html/system/Session/Front.php(270): IPS\Data\_AbstractData->__set('session_ada3748...', Array)
#4 [internal function]: IPS\Session\_Front->write('ada37484a39e7af...', '')
#5 {main}
  thrown in public_html/system/Data/Cache/Xcache.php on line 66