No post mode specified

I am registered, and I have posted successfully before, but not tonight.
I spent hours crafting the perfect response, and then couldn’t submit it. The error message was

No post mode specified

I cannot figure out what I might be doing wrong.

I’ve had plenty of “debug errors” but I haven’t seen that one. I did see that FA was down for a short time yesterday…?

Hmm, haven’t seen that error before. Have you tried posting it again?