MRTG got undef from extrenal script

Periodically monitoring connection to many network devices for few years, using MRTG and mrtg-ping-probe script. It does work fine for long, long time, but in last few days it shows huge packetloss to all machines, but this is not happens in reality.

http://picpaste.com/packetloss-Rljllsk6.png

Monitoring machine load is relativelly normal

http://picpaste.com/pics/load-9tJJr5W3.1403955587.PNG

I see all 'undef' instead of values in debug output:

----------------------
c:\>perl C:\monitoring\mrtg\bin\mrtg c:\monitoring\pingp\packetloss.cfg --debug=snpo
Daemonizing MRTG ...
Do Not close this window. Or MRTG will die
--snpo: run external perl.exe c:\monitoring\pingp\mrtg-ping-probe.pl -t 42 -l 1000 -p loss/loss cisticz.pankrac.czf
--snpo: External result:undef out:undef uptime:unknown name:unknown
--snpo: run external perl.exe c:\monitoring\pingp\mrtg-ping-probe.pl -t 42 -l 1000 -p loss/loss lazna.pankrac.czf
--snpo: External result:undef out:undef uptime:unknown name:unknown
--------------------

But If I start script not from mrtg but manually from cmdline, than expected numbers are on stdout:

-----------------------
c:\monitoring\pingp>perl mrtg-ping-probe.pl -t 42 -l 1000 -p loss/loss lazna.pankrac.czf
0
0


c:\monitoring\pingp>perl mrtg-ping-probe.pl -t 42 -l 1000 -p loss/loss cisticz.pankrac.czf
0
0
-----------------------

This is perl, v5.10.1 built for MSWin32-x86-multi-thread

Perl version is not new, but until now I have no reason to upgrade it, everything worked fine. Anybody have idea, what can be wrong?

L.