I see. I guess that there is a missing \endgroup somewhere? I think this can be quite hard to find :/
Running with etex and \loggingall (specifically \tracinggroups=1, as I recall), the log file will continue with a line like ### simple group (level 1) entered at line 1 ({) Unfortunately it does not report the file (Phelype is contemplating working on that), but at least it would be a glimmer of a clue ... -k