I’ve done some more investigating of my own.
I’m not sure if this is one bug or multiple bugs, but what happens is that when the bug occurs (in my case), the Edit Notification page includes form fields that are supposed to be hidden because they correspond to a different notification type, and as a result they don’t work and they break the page.
For example, the New User Registration – New User notification is only supposed to have 3 fields: Notification For, Subject and Message Body.
But when the bug occurs, other fields appear: Email formatting, Additional Email Fields, From Name and Email, CC, BCC, and Users. Sending a test email doesn’t work, and the shortcodes link neither.
With BNFW the only active plugin in my test install, I can make the bug appear by activating the Decon Character Counter plugin too. That’s the easiest way to reproduce it.
However, I can make the bug appear without having Decon active. What threw me off initially was that with my normal bunch of plugins active, I can make the bug appear by activating the Image Override plugin. But if I then activate the Social Warfare plugin also, it corrects the problem. And, if I only activate Image Override and BNFW and nothing else, the bug does NOT appear.