Running a custom Sensor fails

I have created a Custom sensor as VB-Skript. It checks the modification date of a logfile on a remote systems and returns the message "OK" or "WARNING" with the following code:

IF a > b then
    wscript.echo "Warning"
    wscript.quit("1")
else
    wscript.echo "OK"
    wscript.quit("0")
endif

To access the logfile I use the following method:
Servername\c$\path\filename.ext

When I run the script directly it works fine. When I try to run it via PRTG it gives back the following message: Response not wellformed: "C:\Program Files (x86)\PRTG Network Monitor\custom sensors\EXE\checkSyncServiceLog.vbs(35, 1) Microsoft VBScript runtime error: File not found "

and the sensor is set to error.

What is wrong?

custom-sensor response-time vbscript