Ceon 5276 -- invalid edited data. This suggests to me that there is some string error

Hello,
We are having problems with cards being declined with Sagepay. We have Ceon version Direct 2.0.2 installed and currently 1.3.8a with 1.3.9 update to be completed in the next week.
I have contacted Elavon about the declines and they say that the errors coming through to them are
"5276 -- invalid edited data." They go on to say "This suggests to me that there is some issue with the creation of the data package that is being submitted to the bank for autorisation. Possibly extra characters or insufficient characters being assembled into the data string".
Anyone here know what this means or a setting I may have to adjust to sort this out? This is starting to be costly , so any help would be very greatly appreciated!
Thanks in advance,
B