If it works when you remove QAC, you'll need to find out from QAC dev what it's preventing from running or reading when clients connect.
The ULX motd is a both server side and client side process.
Server sees player connect, if motd is "on", server reads file or URL, sends the data to client, client reads that data, and is supposed to open it.
Seems QAC could break any of that process.