When editing the subject header I also can't type a colon (or paste a string containing one)
This should be an easy fix, we should just override the ex button for that binding context.
I'm still hitting this in recent master (00b9ca503286d5dc43def31db713e853c8aeee13), is it really fixed?
@~jon what's in your ~/.config/aerc/binds.conf? You need to overwrite the $ex binding in the compose mode