Event ID 13 from the NPS source — "A RADIUS message was received from the invalid RADIUS client IP address x.x.x.x." NPS only accepts requests from configured RADIUS clients; a request from an unknown source IP is discarded before any authentication happens. Users see timeouts, and there is no 6273 event because processing never started.
Event ID 13 (NPS)
NPS Event 13: RADIUS message from an invalid RADIUS client IP
What it means
Common causes
A new or replaced switch/AP/controller was never added as a RADIUS client in NPS.
The NAS sources RADIUS traffic from a different interface/IP than the one registered (common on multi-VLAN switches and controller HA pairs).
NAT between NAS and NPS presents a different source address.
After migrating NPS to a new server, client entries were not migrated.
How to fix it
Add the IP from the event as a RADIUS client: NPS console > RADIUS Clients and Servers > RADIUS Clients > New, with the correct shared secret.
On the NAS, pin the RADIUS source interface (most switches support a source-interface setting) so the address is stable.
Migrating servers? Export/import config with
netsh nps export filename="nps.xml" exportPSK=YESandnetsh nps import filename="nps.xml"to carry clients and secrets across.