I have seen occurrences of Asana rejecting email with various messages over the past 5 years or so. I myself spent a lot of time working through one of these situations, years ago, with an error message somewhat similar to yours. The ultimate, fragile solution had to do with some truly arcane, low-level details as to how our corporate email was handled.
There may be clues below as to various failure modes that might be affecting you, but unfortunately I don’t know of a silver bullet.
https://forum.asana.com/search?q=email%20create%20tasks%20error%20order%3Alatest_topic
Good luck!