Three identical cameras, set up identically. Vera gets snapshots from 2, not #3

These are Dahuas. I’m using the same snapshot URL for all three. All three have the same snapshot configuration. Two show the expected preview in the UI5 GUI. The third used to, but got spotty for a while, and then quit. All three can be accessed identically using the browser interface, Dahua’s iDMSS.

I see no errors for these failed accesses in the LuaUPNP.log. The problem camera has nothing extra in its (extremely terse) log. I’m puzzled that the accesses just fail and neither end of the conversation complains.

Any ideas (aside from deleting the camera from Vera and starting over) on how to debug this?