Unable to remove Home Page URL in Admin Area

Hello,

I am unable to remove the Home Page URL that I defined in Admin > Settings > General > Sign-in restrictions.
I cause me trouble (because I am not redirected to the sign_in page when I try to access to mygitlab.com)

When I remove the Home Page URL parameter and then try to click Save, I have a 500 error.

Can you help me ?
Regards.