Your post is unfortunately lacking quite a bit of information:
What is your current Kirby version?
“Suddenly it stopped working”: What does that mean, without you changing anything on the site? Or did you update your site recently? Setting false for the slug is not possible in Kirby 4.2 (currently latest version)
4.1 added that you can set slug to a string template which will be used to create the slug. If that is set, the field is hidden as it’s not needed then.
With that addition, the exception was also added to make clearer when using a wrong value here.