IDOCFlatToXmlConvertor returns StringIndexOutOfBoundsException

Hi,

After upgrading our PI production system from 7.11 to 7.3 file, sender channels with IDOCFlatToXmlConvertor module, return next error:

MP: processing local module localejbs/SAP_XI_IDOC/IDOCFlatToXmlConvertor

Error MP: exception caught with cause java.lang.StringIndexOutOfBoundsException: String index out of range: -4

These channels were error free before upgrading PI system.

What could be the cause?

Thanks and regards