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.

What’s going on

There isn’t a mod limit for Arma 3 servers, what you’re running into is a packet limit. The UDP packet limit is just to maintain compatibility with old routers. By old, I mean ancient, this rarely happens, but I know someone currently deployed that does have this issue. This is one of those undocumented features of the Arma 3 server, and I don’t know why. People will combine mods against many others’ wishes to overcome this limit. I understand but don’t do that, this is a much better solution. There is a drawback that some people in a rare case will need to upgrade their hardware, but Arma 3 is a pretty high-end game to run anyway. It’s not what I call highly optimized. The performance cost is minimal, the mods will change the performance a LOT more. Generally, your “Server.cfg” file is found in the base directory if not specified. If you use Host Havoc, you’ll find the server.cfg in the A3DS directory.
Arma Server