Kimmo,
Of course, your right. This approach would be fine for you or me, where we'd want to go to the cause, get it corrected, and get that bike done to move to the next one. But for the OP, I thought that it would be instructive for him to view the parts as independent parts of the whole. He likely hasn't done a careful inspection and cleaning of the chain, and disassembling / repairing a derailleur in situ can be difficult while fighting against chain tension and the pulley cage. My advice was to break things down to their constituent parts, to inspect, to learn, and to re-integrate, thereby helping the OP to diagnose and correct his issue(s).
I'll now dismount my soapbox, and break it up for firewood. It's cold and lonely here in thoroughville.
Phil