Package: <Install> Version: <11> Fresh Gui install. Two nics on different networks. If I enable one with network manager in gnome, it disables the other one. Re-enable the one it disabled, and it disables the other one?? Turns out this was a graphical install that created file called ens3.nmconnection in /etc/NetworkManager/system-connections. I removed this file and rebooted. Both nics can now be enabled at the same time. A Text install creates a file called "Wired Connection 1" in the same folder. Removed it to resolve. Looks like these files get created if you install a graphical desktop (gnome) during the install. Three different installs, same results. I did a text install with no gui, then installed the gnome desktop after reboot. No issues.
This is a virtual machine install running on an Ubuntu 18.04.1. Kernel 5.4.0-77-generic #86~18.04.2-Ubuntu SMP. 64 Gigs Ram. qemu-kvm version 1:2.11_dfsg-1ubuntu7.37 Debian ISO debian-11.0.0-amd64-netinst.iso size 395.3 MB md5sum 499953266841cae41612310e65659456 sha256sum ae6d563d2444665316901fe7091059ac34b8f67ba30f9159f7cef7d2fdc5b f8a Installs: Debian Gnu/linux 11 (bullseye) x86_64 kernel 5.10.0-8-amd64 shell: bash 5.1.4