MRTG showed strange graphics, "stable", green filled rectangle with blue line above it. Fixing the problems, I had examining configurations, logs, crontab, but even this morning the graphics showed nothing, just white sheets. The problems lied on network cards (eth0 and eth1), but what were they, really?
Confusing problems eventually come from very small, unnoticed things.
That's it, after about 2 hours do that-and-this (not really all the time used only for MRTG), it is found in /etc/hosts:
Corrected to 127.0.0.1 and everything is fine.