The plugin does not sanitise and escape its fields, allowing high privilege users such as admin to perform cross-Site Scripting attacks even when the unfiltered_html capability is disallowed.
Proof of Concept
Create/edit a new 404 via the plugin and put the following payload in the "Please enter the 404 URL in here:" field:
v < 3.4.9 - "><svg onload=alert(/XSS/)>
v < 3.5.1 - " style=animation-name:rotation onanimationstart=alert(/XSS/)//