The "communication error" started to pop up about a year ago on my N900. It took about 2-3 weeks until the phone was completely unusable. I didnt think about repairing it as I thought the warranty was only 6 months. I got an Android phone instead and put the N900 away in my electronic scrap box.
Now as Android starts to annoy me more and more every day I picked up the N900 again, charged it and ... well ... "communication error" again. I went to the operator store and they told there are 2 years of warranty!
The phone is now on its way back to Nokia - hope they will fix it!