This specific issue can be confirmed from with
virt-manager by checking the "Details" (from right-click menu) of the QEMU/KVM hypervisor, navigating to the "Virtual Networks" tab, and attempting to start the the
internal1 network, which will give the error:
Error starting network 'internal1': error creating bridge interface virbr0: File exists
As the last few words suggest, this issue is caused by the
internal1 interface starting to bring up its slave interface
virbr0, but the interface already being up. To resolve it, simply delete these existing interfaces.
ip link delete virbr0ip link delete virbr1
After this, the internal networks can be started, followed by the dependent VM(s).