Unable to add custom app

Anytime I try to add a custom app I get the generic “Something went wrong, we’re sorry”

URL: http://reviewboard/

URL only accessible on internal network but should still forward correctly once setup. Favicon used website favicon.

In previous version this could be done. I have other apps which have a URL like this. It seems that possibly in newer version you are checking to see if the URL is well formed, perhaps ending in .com/.org etc.

Can anyone advise how I can do this?