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It appears that you are trying to open a Protege Fames ontology
(usually saved in .pprj, .pont and .pint files) and trying to cast
it to a Protege-OWL Model. This of course won't work. I suppose
this is due to an oversight on your part.
Just in case you want to convert an existing Frames ontology to
OWL, here are some links with some useful information for you