I've moved on from the bottleneck issue (other thread) -- which means I can't stop focusing on the wifi channel issue.
I have reset the radio provisioning and recreated the wifi interface for 5ghz.
Tried 5Ghz N, AC, and AX
Left the Channel Width empty.
And tried all frequencies from 5160 to 5885 in 20mhz increments. The only 2 that do not produce the error are 5180 and 5745 (even with the channel width left empty).
This is really crazy.
Anyone have any idea what could be going on?