If you had FileMaker Pro Advanced, you could enable the trigger and click this button and watch the script execute to see if any issues arise.
If this button uses Go To Related Records to pull up the related portal record, "nothing happens" is the expected result if there are no related records and you perform GTRR from a portal row. You might double check your relationships and the options specified in Go To Related Records.
Interesting. Even though it had never been moved before it stopped working, all I needed to do was nudge the button one step over and then back and then it worked again. Must have been some sort of glitch.