Hi,
Request you to search SCN before posting as a new question. Its a very common issue and I could find many threads in SCN regarding the same.
http://scn.sap.com/thread/1917065
http://scn.sap.com/thread/797052
http://scn.sap.com/thread/1622371
http://scn.sap.com/thread/747830
These threads already discussed the possible error and the solution. Please check the same.
Regards,
AKPT