First, make sure to update Plutonium: https://plutonium.pw/docs/update/#how-to-update-plutonium-mod
Second, if Plutonium is updated but you’re still facing the error, then the cause of the error is joining a server that’s running a DLC map which you don’t have installed. If you own the DLCs, search your Steam library for “Black Ops”, you should find 2 apps for BO1. Then, right-click the Multiplayer app, select Properties… > DLC, then mark the DLCs as checked.
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LoadedFPS
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