I have enabled the default WP theme and see the same error while trying to save any widget http://recordit.co/uFjgBSGEY6 So, looks the problem is not related to our theme. Contact your hosting provider and ask them to check the server error logs and fix the issue.
Or enable old widgets in Theme Options > General and use classic widgets instead of the updated one https://prnt.sc/1wubx9t
