If you're using ALVR it will disable other registered external SteamVR drivers by default.
After restarting SteamVR, KinectToVR should show "Sucess!" in the Kinect Status label.
This means the driver is missing from
openvrpaths.vrpath, the file that SteamVR reads to find various important directories, including external drivers.
Press + R to open the Run dialog, and paste in
%localappdata%/openvr then press Enter to open the folder in a file explorer.
Download this file: openvrpaths.vrpath
Drag it to the folder, and click yes to overwrite the existing file.
After saving the file and re-opening SteamVR, go back to the top of the page and make sure the driver is enabled. Then launch KinectToVR again.
If the driver is enabled and you restarted SteamVR after enabling it, and you're still getting the server error. Try checking if you have third-party firewall software that could be causing KinectToVR to be unable to connect to the driver.
If you have no such software. You should come ask us for help on Discord.