We are using WFS-T to edit a table with a geometry of type MultiPolygon. When the debug level of the geoserver is set to PRODUCTION or DEFAULT everything works great.
If we set the debug to VERBOSE the inserts or edits fail when using a multipolygon with more than one feature (with a multipolygon of 1 element works fine).
The attached file contains the exception trace.
If you need further details, please ask.