I raised a ticket with Evernote, they responded very quickly, here's the response:
'I'm writing to let you know our developers have found the root cause and they're currently working on a fix for the issue you reported. While the version with the fix is not released, a workaround is to make an edit to the note where you're creating the task. It looks like this will "break" what is triggering the error message and resolve the issue temporarily.'
I had been creating tasks directly from Tasks, now I'm inserting them into Notes, and that is working fine for me