From the information provided it seems that it fails to find the Access Control system in the event server, which is rather odd.
I would say try to refresh the configuration of the system from the management client and see if that succeeds and if the login is successful after. Unfortunately if the login fails, then the only troubleshooting can be through the logs, and it seems you’ve tried that already.
Refresh configuration did not help. This seem to be a permanent problem reproducible on multiple 2025 R3 systems.
Attaching the ACUnit configuration from one of the test systems that have this problem (“DEV: Info” tab). We do define a “system”. Anything that looks wrong?
Everything seems to be as it should, other than it’s not necessary to define both a Server and a ACUnit to represent an AC system.
The only thing that I think you can try is to remove and add the system again. I also wanted to ask if you tried using an R3 Smart Client and if that was working.