If you have FMP Adv. Open the file with the debugger tool running and use the debugger to abort the script.
If you have not disabled user abort, open the file and quickly hold down the esc key(windows) or command-. (command and period keys, Mac). That should abort the log in script.
I'd forgotten the debugger in my panic. That worked.
I'd tried command-period. Possibly because the offending line was the first one in the script, it didn't abort the script in time.
At any rate, all is well.