FYI - I found the underlying issue:
https://issues.jboss.org/browse/ISPN-2182
Ah, yes, the legacy --> current config transformation looks like the biggest non-tested stuff ever. ;-)
As I've previously had to fix a few similar, if not almost the same, issues before - wrt indexing.
A workaround for the issue, as well as the complete 5.2 upgrade can be
found in this branch:
https://github.com/pferraro/jboss-as/tree/infinispan
Shouldn't this be fixed in Infinispan directly?
(as that's where I had to fix my indexing config issues)