Conan Exiles Dedicated Server Launcher (Official Version: 1.7.8 / Beta: 1.7.9)

Possibly the information is somewhere in the logs for the client and the server.

There could be multiple reason, but the main one is to know if the server actually detects the connection attempt and just reject it. Normally with the current moding system you need to have the exact same mods on the client and the server, at the exact same version.

The way that works is that the server compares the list of mods the client has, and if it does not match it displays a mod compatibility error while returning the list of mods that need to be updated, then the client disconnects, updates the mods, restarts and tries to join again.