SKD-05VDC-SL-C wont turn off with remote control


I have a problem with my mega 2560. I would switch a SKD-05VDC-SL-C with a infrared remote control.
I have a lamp with a 12VDC external source. The remote works when I try the program with a led or when I let the relais blink with the lamp (but without the remote). The program and the remote also works when I unplug the primary side (external side) of my relais. Then I can switch the relais on and off with my remote. The problem occurs when I plug in my lamp. Then its only possible to switch on the lamp. But I can't switch it off.

Hope you can help me!

and the program is… ? (with a drawing of the wiring and power supplies)