you can search for the string/part of the strong you get in error message. Probably it will be in BMA/BSM files. After finding the error in the file, check the events/messages prior to this. One of the message can tell you why backup goes through full instead of INCR/DIFF.
Also Check how to get patches(703) and apply them. Then check if everything works fine.
Fyi: Also DP has released major release 8.1.