Updating the provisioning profile for my app, one of my testers has an iphone 6 with a 41 chars UDID, and only 40 of them are taken.... so the app eventually is never installed in his telephone.
UDID for provisioning profile is 41 chars long and is not taken
41 seems like an error/miscommunication.
Ask them to find that device in iTunes, then click on the S/N to toggle to UDID, then right mouse click on that string to copy. Pasting that into a email you can check.
That's exactly what he did, and he gets a 41 char string instead of 40
I'm finding reports of devices w/41 going back to 2009, but they were dealt w/by using Safari to add them to the member center/devices count (internal testers vs. external).
I'd guess that in your example, and if this becomes more common, the process may need to do some catching up.