This error is generated by the Cerber Security Plugin. You can add an exception on ‘Antispam’ under the WP Cerber menu, ‘Adjust Antispam engine’, ‘Query whitelist’. You probably want
{\/contact-us\?enquiry_type=\d+}
for a regular expression to match any numeric enquiry_type value. The relevant documentation is here: Configuring exceptions for the antispam engine.