I can’t be sure exactly what the issue is, but there has been a recurrent issue of email failures that is quite similar to this. See Creating tasks via email gives error "We couldn't process your email." for most users - #6 by Marie for an example. The solution was not an easy one and it didn’t really work that well.
I would recommend creating tasks from the Gmail add-on for Asana, if that is an option for you. It works well, never fails, and allows more options at the time of task creation. Best practices for a 1-2 person start-up - #2 by Ellen_Dapper