"Vaultmp.dll" and "can't connect" to server errors

Hi,my friend downloaded the server files and client files in order to open a server. His server was in the list, but when i tried to join a "can't connect to server" error just popped.And he(my friend) can't join his server because an error pops too: "vaultmp.dll error".
please help us.:)
thank you very much.

Reinstall the Server, and delete any old version server files if he got them.

And he should also clean hes own VaultMP client files for old versions.