When a serialization error occurs, we raise XMLRPCMarshallError, regardless of the protocol being used:

[tbabej@vm-206 ~]$ ipa -vv user-mod random --principal-expiration="2013-02-02T12:00:03"
ipa: ERROR: error marshalling data for XML-RPC transport: datetime.datetime(2013, 2, 2, 12, 0, 3) is not JSON serializable

Note that we refer to two different protocols in the same error message, which can lead to some confusion.

The error message should be go along the lines of:

ipa: ERROR: error marshalling data for JSON transport: datetime.datetime(2013, 2, 2, 12, 0, 3) is not JSON serializable