Sorry for the troubles. Unfortunately with our limited resources we can only test and support the Firefox, Thunderbird, and Seamonkey platforms.I'm not sure what the differences are in PaleMoon (though my understanding is that it has the same underlying engine as the Mozilla platforms).
One possible debugging avenue might be trying to add ReminderFox into a Firefox install and trying to import your existing ICS file from your PaleMoon profile just to see if that works on Firefox or gives the same kind of error. That might narrow down if it's a ReminderFox issue with the particular data in your file, or something with the underlying platform itself.
Thanks,
Tom