It could be a problem with that hallway light fixture. I would remove it
and put a temporary pigtail socket in its place and go from there. I
assume
the GFCI trips when the light is activated from either switch.


Most likely, "someone" used the "wrong" neutral when wiring the lamp.

It's not unusual to have more than one circuit appear in a box. It the
hall way fixture picks up a neutral that doesn't pass through your GFCI as
soon as you complete the circuit the GFCI on the "hot" wire will trip.

You can chase that down or just forget about running a string off one GFCI.
The GFCIs are relatively cheap. Just put in a GFCI for each outlet you
want protected and leave the string that goes to the hall light and switch
loop "unprotected."