Haven't seen any such issues reported. Past issue reports with other OS versions suggest that you may have a dictionary file that is damaged or missing.
Is my only choice to reload Filemaker or is there a simple way to replace the dictionary file?
We don't even know for sure that the dictionary file is the culprit, just that it's a possibility. Before you go that route, create a small test file from scratch and see if your spell check options are still greyed out.
Replacing the dictionary requires that you have a valid copy with which to replace it. A re-install is one option. If you have Filemaker successfully installed on a different computer, you might be able to copy this file from there.