The next time I tried to clear the bug by restarting FileMaker, it failed to work. I had to do a hard system reboot to restore it to normal behavior.
The next time I press a button that runs any script, the Script Debugger window opens. I can close it, but it displays for every script that I run after that. Closing the application window and restarting FMPA appears to be the only way to turn off this behavior.
Have you checked "pause on error" ?
Pause on Error is not checked when I open Script Debugger now. I do not remember whether I had it checked before rebooting.