There's an issue in the latest Linux kernels shipped by Ubuntu in 16.04 that is causing users to get 'PORT_ERROR' messages on their ZeroTier networks starting with kernel version 4.8.0-49. It boils down to a bug in the kernel that doesn't allow the setting of MTUs greater than 1500. ZeroTier uses an MTU of 2800.
This issue affects more than just ZeroTier users. If you wish to track the issue, you can find it on Launch Pad here: https://bugs.launchpad.net/ubuntu/+source/linux-hwe-edge/+bug/1679823
The only known solution as of today is to downgrade the kernel to 4.8.0-46.