Raspberry Pi 4 running Ubuntu 20.04 can't connect to some WLAN APs

I have a desktopified version of Ubuntu server 20.04 on a Raspberry Pi 4, now running NetworkManager. The wlan0 interface is using the built-in Broadcom wifi chipset.

I am able to connect to some WiFi APs, but never to some others. On those APs that fail, I continuously get the popup asking for password, and authentication fails every time.

There is no issue with the AP password, I can connect to it from my Windows laptop and phone. An ethernet cable connection to the router works fine from the RPi-4. The RPi-4 connects to other APs and mobile hotspots without any problem.

What I have found so far is that connection always fails for Channel 6 (2.437GHz) and not on channels 1 and 5. Unfortunately, I don't manage the Wifi router in question, so can't change the channels there.

I found another user facing a similar issue on the Intel platform, but that didn't help:Cannot connect to wifi in Ubuntu 20.04

Thanks for any pointers. I have the output from the Ubuntu wireless-info script, but can't post inline here because of the size.

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