Could you try the same cumulative dump to the "compress::" module and see if it gets the same error? That would help establish whether it is a general issue with API modules or something more specific to your 3rd party one.
Backupserver Traceflag -D16 (trace rpc events) may provide some additional diagnostics. (reboot backupserver with -D16 added to the parameter list in the RUN_BS file).
-bret