I found kind of a solution, downgrading de JVM from 1.6.29 to 1.6.27. However some of my colleagues have no problem when using 1.6.29 so I still don't know where the real problem is. Any ideeas on how to get to the real problem are still welcomed however I can move on.
Another tested working solution may be to use the jTDS driver instead of the one from Microsoft, but this was not an acceptable solution in the long run for me.