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The error sometimes stems from your ISP blocking P2P server lists. Try using a just to fetch the server list, then disconnect once you’ve joined a race. Are you using the or Content Manager
This article walks through causes and step-by-step fixes for Assetto Corsa (including Assetto Corsa Competizione where applicable) showing the error “the server seems to be unavailable” or otherwise failing to connect to multiplayer servers. Follow the sections in order — start with simple checks, then progress to router/PC configuration and advanced troubleshooting. Are you using the or Content Manager