When I join to a server and I have never downloaded the mods that are used, everything works fine.
When one of the mods got an update until next time I join the server, it will start downloading but will stuck during the process. The server config is set with blank version so that the newest will be used.
The only workaround to join the server is:
Go to the download page of your workshop, manually update the mods and then join the server.