Or: How Oracle screwed up and how to fix it
A few days ago, oracle has updated the java runtimes to 7u51. Starting with this update, self-signed webstart applications are completely blocked in the default security settings. If this is the case for you, you'll see this screen (for the custom clients, for the vanilla client the name and location will differ):
Sadly, oracle decided to screw up independent and low-budget (or non-profit) developers with their latest update. To get around this issue, you open the java control panel: javacpl.exe. Make sure to open the right control panel (you'll have a different one for each java installation you have). In the control panel, go to the security tab and lower the security to "medium". (notice how oracle implies they don't have an option for low security? wishful thinking, I say)
Additionally, with the most recent versions of Java you also have to include the download locations as 'trusted':
http://forum.salemthegame.com/viewtopic.php?f=10&t=8388&start=20#p167676
This will once again allow you to launch the clients (both the default and the custom ones)