Airport Utility accessing settings on AirPort Extreme TC

I have two Airport Extremes, both the last versions, one is a Time Capsule version. They are configured as a roaming network. It works well. BUT after one of them has been powered on for an hour or so, I cannot access the settings on TC AE using Apple Airport Utility. I can see the TC AE OK, but it asks for a password which when entered has no effect. The other AE behaves as it should when accessing the settings with Airport Utility. If I power off and on the offending TC AE, I can access the settings for an hour or so and then it goes back no access with no interference from me.

Both AEs have IPv6 set to Native. I have powered down the whole network and powered on each component in turn. I have factory reset the offending TC AE. The same symptom occurs on iOS and Mac OS. All components are on the latest software and firmware. Tried it also on High Sierra and Catalina - same result.

I’m puzzled! David