Customer has an error message when he try to add a device on a master LMS 2.6
It use to work find, here you can find a screen shot.
As I said, the two servers have different MDF versions. The slave has 1.24, and the master has 1.22. On the both servers, go to Common Services > Software Center > Device Update, and check the box next to Common Services. Download and install the available updates. That should bring you up to MDF 1.28 on both servers, and this problem should be fixed.