On Windows 10, FileMaker 17:
When using the debugger to step through a script containing an Open URL script step where that step attempts to run an fmp:// url and fails, returning an error (e.g. "fmp://$/myFile?script=missingScript¶m=12345" returns error 104 ), then the F5 & F6 function keys stop responding for stepping in the debugger.
1. Open the Script Debugger and run a script containing an Open URL script step that errors out (note: not sure if it's specific to attempting to open an FMP URL or not, but that's what I was doing, as explained above)
2. Use the F5 or F6 function keys to step through the script
3. Notice after the Open URL script step gives an error, the aforementioned function keys stop responding for the duration of that debugging session.