Troubleshooting for Agarose Gel Electrophoresis
I haven't found a manual for this machine, but my guess is that you exceed the maximum current which the power supply can provide. This is something I have experienced myself for electrophoresis. 

Usually power supplies then switch into the constant current mode at the maximum current they can deliver, which I think is what happens here. If the voltage you choose is lower, you do not run into this limitation, thats why it is working without problems.

So you can either run your gels at constant current, which makes them run faster but also produces more heat, which can make your gel blurry. Or you live with a slightly smaller constant voltage, have your gels run a bit slower, but probably have a bit better resolution. I have to say that I have never had any problems with the resolution, even when I ran my gels really fast.