I’d love to know what this problem is. I don’t just say that in the “if you tell me there is a problem, tell me at least something about the problem” way, but I’m very curious about the technical details of it.

It can’t be that bad, because force-closing and reopening Mail.app fixes it. Usually it seems to occur when you have a spotty connection and Mail.app isn’t able to download the actual body of the email. If they can detect the problem – as demonstrated by the error message – and clearing some cache (or whatever other voodoo force-closing achieves) can resolve it, why can’t it be fixed in a non-shitty way?