The RMTxStore implementation does not work with Derby versions 10.8 and higher due to a bugfix for
https://issues.apache.org/jira/browse/DERBY-4965. The issue can be reproduced by replacing the Derby version in the parent pom.xml to 10.8.1.2, afterwards some of the existing test cases for WS-RM fail.
The fix for this is rather trivial (and proposed in the Derby 10.8 release notes). I attach a patch for the trunk to this issue.