I get it frequently, usually takes me 5 or 6 tries to get into the game after 5pm GMT. My guess would be the server is struggling with the volume of people, it happens all the time during the early stages of an MMO's lifespan so nothing to worry about.
So far I've been unable to complete ANY missions outside the introduction ones because I keep getting disconnected. Seriously, I have about 8 missions in my log and NONE of them are doable because I get dropped from the server along the way and they're reset.
Looks like I'm limited to grinding out "secure sirius block" repeatables until Lt Commander.
Is there no way Cryptic can improve the missions in STO so that it saves your progress should you quit or log out? It's very frustrating having to restart the whole mission from scratch.