Test on a totally new World using Navezgane with only the Server Side Zombies PLUS mod. If they are spawning or can be seen in the F6 menu it tells us it is loading for you appropriately. Add back in any other mods you are using one by one until you cannot spawn any as explained in the post. It may help show a conflict or a mod that is overwriting the Server Side Zombies. A full copy of the whole logs file uploaded to somewhere like pastebin (as recommended for sharing bugs with the game devs) can share things that don't always show in the console.
https://pastebin.com/
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Test on a totally new World using Navezgane with only the Server Side Zombies PLUS mod. If they are spawning or can be seen in the F6 menu it tells us it is loading for you appropriately. Add back in any other mods you are using one by one until you cannot spawn any as explained in the post. It may help show a conflict or a mod that is overwriting the Server Side Zombies. A full copy of the whole logs file uploaded to somewhere like can share things that don't always show in the console.
https://pastebin.com/