"File not found." instead of custom 404 file with php-fpm enabled

I found that badfile.php produces "File not found." instead using the 404 error file designated in .htaccess when php-fpm is enabled. I found the solution here:

SOLVED - Getting file not found vs 404 with PHPFPM

WHM Home » Service Configuration » Apache Configuration » Include Editor
Under the "Pre Main Include" section, add: ProxyErrorOverride on

But that breaks webmail.domain.tld giving this error:

"Access Denied
This server could not verify that you are authorized to access the document requested. Either you supplied the wrong credentials (e.g., bad password), or your browser doesn't understand how to supply the credentials required.

Additionally, a 401 Unauthorized error was encountered while trying to use an ErrorDocument to handle the request."

Anyone get this sorted...?

 

Thanks. It just seems like there should be as more universal solution, or that the above solution shouldn't break webmail. I'll post back if I find anything.

 

In glancing at that page I don't see the connection. I'll have to give this further consideration after I get back in the office week after next...