This is really strange, but I replied to a student question received via a canvas message via my own e-mail, and it gave me an error message telling me I hadn't attached a file to the message. This is correct, I hadn't, but there was no reason for me to have done.
I was able to override this and send the message anyway, but it's an odd bug. Anyone else run into this or have any idea why canvas suddenly thinks I should be attaching files to my replies to student messages? Actually, even if I had wanted to attach a file, it wouldn't have worked, because my campus e-mail system doesn't support sending attachments to replies to canvas messages that I get when I check my e-mail (rather than logging into canvas).