Commenting out the ISPConfig line in the apache did the trick and stopped the use of the logfile.
Too bad though that it is PHP again that causes this mess. Daily logfiles of the webserver reach about 1.7gig and i assume they will give the same problem when they reach 2 gig.
We never had these problems on Plesk, with daily log rotation... maybe it's interesting to find out how their logsystem works because i can imagine others who use ISPConfig as well find this error very annoing.
Thanks again for your time.