Ask questions about dedicated servers here and we and other users will do our best to answer them. Please also refer to the self-help section for tutorials and answers to the most commonly asked questions.
When copying from one server to another, the chance of a corrupt file or missing files can happen.
When you updated, did you also use -verify_all in your update parameters?
If you have any plugins, try disabling all of them and test it.
If it goes away, then start adding the plugins back in one by one until you find the plugin that causes the error.
Then reinstall the plugin to see if it goes away.
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Since gmod came out with a couple of new updates in one day, it is possible that they broke your server. You may need to update your addons/gamemodes/plugins and gmod again.