You can try enabling logging as suggested here: https://www.coderesort.com/p/epicode/wiki/404Handler/DocCms6#Custom404HandlerforCMS6. See if you can capture an error in the logs. If that doesn't help, you can try raising a ticket from the same link and see if you can get assistance that way.
We have installed the BVN 404 Handler on a CMS 6 R2 site and are using the Custom Redirects Manager gadget to add URL redirects, but there is a problem with the Suggestions tab. The tab lists a number of 404 URLs but it is not possible to add new URLs for these. Nothing happens when pressing the 'Add' button. No error message is displayed, the button is simply not doing anything.
Any ideas why this occurs? Could some files be missing from the installation? The gadget seems to work except for this behaviour.