your problem looks exactly like following behaviour  :

"readme.1st" from adsm client 3.1.0.3

"  Known Problems and Limitations "
" ------------------------------ "
" When backing up already-compressed files with a Version 3 client "
" to a Version 2 server, under certain circumstances you may "
" receive error message ANS1311 and some of the files may not back up. "
" Setting the COMPRESSALWAYS option to NO on the client, or "
" turning off server caching, will allow the files to be backed up. "

You have V3 Server, but the problem seems to persist anyway.

The quoted part of readme.1st 
can never be found in readme.1st from tsm3.7 client,
but still is there in readme.old - so apparently
all 3.1x version might share this problem,
but not TSM3.7.

Can somebody confirm this assumption?

Juraj Salak
Keba, Linz, Austria