strange problem with RRD graphs

hi, i have installed the lastest stable release of xymon (previously using hobbit) and added my old config files, then added some new host to be monitored and all ok for a few days.

Suddenly today i've added a new host on bb-hosts, all was working fine except that all the graps reported values of "nan".

Tried to remove, then delete with drop host command with no solution.

Then i tried to monitor another host -my own pc, just ping test-, added to bb-hosts and got a bad suprise: same problem: graphs reporting "nan".

The strange fact is that all other "old" host has good graphs still reading and showing correct data.

Please help :) thanks in advance.

Alessandro