Hello,
if there are really 40GB of logs produced by this, I do not really believe in only "1 user, 15 businessroles to be removed" 
This looks like a pretty big procedure. Might be the case that the Dispatcher is running out of memory
Suggestion 1: Do it in smaller chunks. Only remove one role at a time and do not do it via the UI but the Management console
Suggestion 2: There are specific Dispatcher Options you can set to increase heap size Memory. By using a Job in the management console you can make sure this Dispatcher is executing the procedure.
-Xmx4096M for example sets the heapsize up to 4GB for one Dispatcher. For further Information you can check SAP note 1347301.
Probably makes sense to combine the two suggestions though.
Regards
Tobias