Dead outlet. GFIC breaker trips unless another breaker is off

I am trying to find out why my outlet in the upstairs bathroom doesn't work. It seems to be the only load on a GFIC breaker in my panel. It will re-trip immediately. However if I switch off the breaker that controls the lights in that bathroom and another room, the GFIC breaker will hold and the outlet works fine. I've tried having everything off and it's the same result. I'm not an electrician but I am pretty handy. Would this be possible because of a failed device or does it mean I have a short somewhere?