I have an exterior GFCI receptacle that failed. I bought a new GFCI to replace this one. When I opened the outlet box, I discovered that the ground was tied both to the ground screw on the receptacle as well as the neutral plate. I thought I would correct this with the new GFCI receptacle, but ran into issues. The entire circuit did not work unless I connected ground to neutral at this receptacle. There are 4 terminals on this circuit, an exterior light, and interior ceiling fixture (both on wall switches), the exterior receptacle I was replacing and an interior receptacle. The GFCI does not feed any other terminal on this circuit. Both switches are in a single box, and the feeder line to the GFCI receptacle is also in this junction box. The interior receptacle was tied in from the GFCI source. The line to the interior ceiling fixture is old 2 conductor Romex with no ground. All other wires in the switch junction box are Romex with ground. The GFCI receptacle, and the two switches are wired parallel to each other. The wiring in the junction box is fine, i.e. the grounds are tied together, the neutrals are tied together, and the source hot is tied to hots that go to the switches and to the GFCI receptacle.
I would like to know what the cause of the behavoir I am seeing is. Is it an open neutral? How would this problem be fixed? Thanks.

Sure sounds like an open neutral in the circuit. The only way to find this is to open each box as you follow the circuit from the panel,and check the wiring and connections until you find the open neutral.