Orbi 6E Router RBRE960 only shows error on Access Control page

Just replaced my RBR50/RBS50 setup with a RBRE960/RBSE960 setup. Things worked pretty well. I did everything step by step over something like 6 hours, bringing my switches and devices online in sections. Among other things, I create reserved IP addresses for ~25 of my network infrastructure devices (switches, PDUs, KVMs etc. that I need to have access via their fixed IP) and make sure each of the ~40+ devices have proper name/brand info put in on the Attached Devices page instead of whatever the router decided they should be, usually wrong and not following any naming convention.

Anyways, as I was doing that, the Access Control page (under the Advanced/Security menu on the admin webpage) was available, showing me the devices as I added them, split between what is currently connected vs. not. Cool.

Then, after about 30 devices, as most of my network came online, that page started showing a blank grey page with a large "earmarked document page with an X in one corner" icon in the middle of the page. What? When I move my mouse over that grey area, it gets lighter and the sentence "192.168.1.1 didn’t send any data." appears under that icon. Rebooting the router didn't work. Trying from different PCs didn't work.

What is going on?

I wanted to telnet into the router to see if any of the environment variables holding modified device info is corrupted due to whatever, but seems telnet is blocked on this router. I posted my question about that separately.

Given the hours I spent for 25 IP addresses, 40+ device entries, and everything else about all the other settings, I am not eager to reset and restart. I don't think a Backup/Reset/Restore would fix this; any corruption will just go into the backup and come back.

Anybody in Netgear want to have a backup CFG file to see what is going on? I feel this is caused by something that might need to get fixed in a future firmware update, soon.

Thanks

Tuna

PS: Also, maybe tellingly, the ORBI app on iPhone was showing 4 disconnected entries with MAC addresses that were non-existent. Turned out, they were MAC addresses that my iPhone/Watch/iPad pretended to have before I turned off Apple's stupid "Private Wi-Fi Address" feature for my home networks. But 4 such fake MAC addresses remained in the history as currently disconnected devices. Since I couldn't use the Access Control page on the router to delete them, I used the ORBI iPhone app. Guess what? I was able to delete 3 of the 4. Deleting the last one keeps raising an "Error Unknown Error." alert. I bet that is the culprit, but I don't know how to get rid of it without a functional ORBI app or TELNET...