I have the same error
ERROR [2015-30-03 21:09:33]
User: unknown; IP: 126.96.36.199
Code: CRYPT_CELL_IS_NOT_DETECTED (1311)
Encrypted cell for data with prefix '"'
AP'"' is not detected
There were no changes to configuration at all, last payment we received was 2 days ago. Lost a couple of orders before we realized something wrong with xpayments.
I'm going to upgrade from 2.1.1 to 2.1.3 to see if it change anything.