I just plugged in the same info and it filed successfully for a VFR flight plan from KFXE to MYAT at 7,500ft, departing in 30 minutes. Now if I could only actually *make* the trip...
It sounds like it's coughing on the 7500ft altitude, but you might just verify your equipment codes and other aircraft parameters to make sure everything is ICAO compliant.
It's been a few years since I flew to the Bahamas, but you used to be able to depart without a flight plan but coming back is a different set of rules. Flight Following from Miami Center is always an option, too.