[Xymon] Holes in xymon graphs

I have a problem with holes in xymon graphs (see attached pdf): The values for load, processes, users stopped showing up at 4.30 this morning. All other values disc, memory, cpu load, network are shown. The client is sending its data to 2 xymon servers, both show the same behaviour. Restarting the client makes no difference?

Any tipps?