I assume that's your customization of the phrase, so it's a XenForo-generated message.
Unfortunately, I don't really have a great suggestion for while the same file wouldn't upload "normally" though upload successfully when dropped in -- especially not spontaneously. There are some differences in the process, but they're generally similar; the requests should be very close. It may be worth seeing if you can reproduce it on a test setup on the same server; that would allow more extensive debugging.