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2.1 no longer works. However, you can still use 2.3 as long as you also use the Nightly version of ScripthookV/DotNet. The only way to prevent the mod from crashing is to disable all scenarios. This will mean police/military will not respond to the outbreak, but it will still allow you to play with the mod.

I don't know if the author is still around on the site, but I think the best thing to do since he's no longer modding is to release the code for the mod and allow others to continue where he left out. Zombie outbreak simulators are a rarity for any game these days, and it would be a shame for this to eventually stop working.