Arma 3 servers can be tricky to configure. I know as I run one. Out of the blue, your server says “Server can’t transmit all data; list of required mods might be incomplete”. You’ve run into the Arma 3 server mod limit, but there’s a fix for that. Here’s what you need to do:
Locate your server profile and find the file called “server.cfg”. Edit that file in any text editor and add this line to the bottom: steamProtocolMaxDataSize = 2048;
Then save. If the server still has the same issue, raise the number again.