Seems patch v1.12.1 broke the default mode of the launcher (or rather, the game now has a check in place whether it's being launched from the official launcher).
For now you can use the launcher's cached mode to launch the game. For that you'll have to add -usecache to the launcher's launch parameters (see instructions in first post of the thread) and, since it currently cannot generate the cache file itself, you'll have to download it from here: Snib's Dreadnought Steam Launcher.cache and place it into your %TMP% folder (yes, that's a valid path, just type it in like that). Make sure your browser doesn't rename it.
The cache will remain valid until the next server maintenance, so typically the next patch. Until then I'll hopefully have an update to the launcher for you (waiting to hear from the devs whether they blocked it intentionally though).