I'm not having this problem, but Steam forums are regulary flooded with topic "Game won't start" and this is very important to fix, because tons of newbies are not joining the game because of that. This problem is mostly fixed by disabling Steam Overlay, but it needs to be fixed anyway. Without fixing that Steam users without overlay cannot buy energy etc.
Most of users are describing the crash by that game starts, screen is black for some time and then crash. This happens everytime, so they cannot play the game!
If you think it isn't important to fix, check this - 15 pages with "Crash" word and mostly this is the problem.
I'm suggesting this to fix ASAP.
This has been discussed and determined that it is more of a steam problem than SK. The most OOO can do is complain to valve about people not being able to use the overlay.
That being said, you can always attempt reinstalling and checking to make sure things are updated correctly.
http://forums.spiralknights.com/en/node/94148#comment-856561