Sending without blocking Outlook so that it is unresponsive means that we have to cancel the original send, do encrypt / sign and then invoke the send once the crypto is done.
It has been reported by a user that this crashes reliably for him. While this is very stable for me and for other testers of the beta.
Currently there is an option to disable async crypto because of this. But we need to figure this crash out before we can finally release this change as default.