Zotac ND22 Frodo 12.1 + XBMCbuntu + Transparency skin: X windows error

Hi Team,
not sure if this is the right place to post this general error I keep getting in the .xsession-errors log file.

------ start of error file -------->
Killed
XIO: fatal IO error 2 (No such file or directory) on X server ":0"
after 1402 requests (1402 known processed) with 0 events remaining.

<------ endof error file --------

Any ideas what is causing this ?

cheers,
dg