If running the server, and those little PC-Icons at the modules ain't blue - you might change their service state first.
For instance, when the server with the necessary services are running (Agent Server, Shard Manager and Gameserver), only privileged accounts (with IP privilege and GM rights) may enter the game, while others (normal users) may not.
On the other hand, if no service is running, but just the modules, no one can enter it.
On another case, I have 2 identical clients, one being stored on a HDD, the other one on my SSD.
The HDD one runs without any problems. The start button appears, no msg, no crash, nothing.
The SSD one does not run without any problems. The game starts, runs correctly, logs in, and so on - but only when using a multiclient tool. The launcher seems not to cooperate somehow from the SSD partition. The start button won't appear, no matter what.
This might be also a considerable issue, which most people overlook because they don't know it.