amp from your forum logs:
flarum.ERROR: BadMethodCallException: Method Illuminate\Validation\Validator::validate/^(?!.*\b(? does not exist. in /data/host/skeleton/vendor/illuminate/validation/Validator.php:1534
I assume that you have provided an invalid regular expression somewhere in the Masquerade config, or something else is not configured properly. Could you please double check that everything is valid (e. g. through something like https://regex101.com)?
Edit: I prefixed one of your Masquerade validation rules with regex:
, could you please check if it works now?