warning message when i use w3 total cashe plugin with xcashe opcode

i have a wordpress blog with xcashe installed and w3 total cashe plugin , the plugin works good with disk enhanced as Cache Method but when i enable it with xcashe it works good for a while and then starts to get warning messages for some pages not all pages the messages is : Warning: xcache_set() [function.xcache-set]: XCache var cache was not initialized properly. Check php log for actual reason in /home/..../wp-content/plugins/w3-total-cache/lib/W3/Cache/[login to view URL] on line 42 i need to solve this warning