The DB2 instance did not have all 4 sizes of Temp tablespace defined.
Resolving the problem
To resolve the issue, the DB2 instance must have all 4 sizes of Temp tablespace created: 4k, 8k, 16k, and 32k. In addition, these must be set as System Temp tablespaces, and not as User Temp tablespaces.
and BPM started happily.