OK, it turns out the reason why this was failing is although the redline layer was still in Collector, it had mysteriously vanished from the web app itself. Why it did not also disappear from the Collector app, I don't know. But I found that it was missing and added it back in, and now the attachments are posting. It would be useful to have more... useful error messages as to what went wrong instead of the equivalent of "sorry, didn't work" because it can take forever to track down a random issue like this.
