For the last 3 days I've been getting this error and kicked from my Realms server I bought about every 5 to 20 minutes.

Things I've tried to fix the problem:

Java Control Panel > Check box for "Enable the operating system's restricted environment (native sandbox)"

Uninstall / Reinstall

Windows Firewall allow all private

Flush DNS

I'm playing PC Java Edition realms on version 1.16.3.

Update: Uninstall / Reinstall after Flushing DNS did have some effect. I did get to play for a few hour span but the problem still comes back after a few hours.