This solved my issue: How to turn off rootless/System Integrity Protection on Mac: Disable SIP: Restart/Power on your Mac (Apple > Restart / Shut Down). Recovery Mode: Hold down Command-R until the Mac logo appears  Wait for OS X to boot into the OS X Utilities window. Choose Utilities > Terminal. Enter "csrutil disable." Enter Restart. Reauthorize extensions as prompted. Reboot. Test and verify.

This solved my issue: How to turn off rootless/System Integrity Protection on Mac: Disable SIP: Restart/Power on your Mac (Apple > Restart / Shut Down). Recovery Mode: Hold down Command-R until the Mac logo appears  Wait for OS X to boot into the OS X Utilities window. Choose Utilities > Terminal. Enter "csrutil disable." Enter Restart. Reauthorize extensions as prompted. Reboot. Test and verify.

I'm having the same issue on my iPhone running iOS 14, but it's working fine on both of my iPad's running iPadOS 14.

FYI, resetting my network settings did NOT solve the problem.

FIXED: I just went to a game that I had previously used with Game Center and signed back in. Boom!

Same here. Everything was working perfectly fine with beta 4. I’m able to login, but then I get a kernel panic shortly thereafter and macOS restarts. Installing a fresh copy now.

Confirmed that "All you have to do is disable "Develop tab -> Experimental features -> VP9 decoder", and then restart Safari" fixes the issue. Thanks, @Arun_Kudiyal.

I am having the exact same problem as the OP even with Big Sur 11.0.1 that was just released to developers today. I can copy and paste from my Mac to my iPhone, but not the other way around. Same complaint. I copy and paste with my authenticator app which I can’t do anymore.