Can't edit draft email created from stationery

I have several draft emails that I created from Mail's stationery. Most of them act fine, but there is one that I cannot edit. I have tried moving it around in my Mail folders and deleting and recreating it, all to no avail.

This is a new computer and I imported all my mail, apps and settings when setting up the Mac. This is the first time I have tried to edit this particular email on this computer. I edited it with no problem on my previous Mac mini.

Does anyone have any idea how to solve this problem? Thanks in advance for any help you can provide.