apache service :php exec() not working

[spacewalker]

Hello,

I'm running xampp 1.8.3 on a Windows Server 2008R2 machine.
I have connfigured apache to run as a service and I use (for testing) the local Administrator as service account.

My problem is:
The PHP function exec, system, passthru don't work correctly:
If I start "7zip.exe" using exec() I get Errorcode 1 and 7ZIP stops.
According to 7ZIP homepage the errorcode 1 could mean that a file is locked - but I dont think this is the case here because:

When running apache NOT as a service, everything works OK

The 7zip command has no GUI, it's just a commandline that I try to execute, something like:
7z a -tzip -mem=AES256 -p123 -mx0 "C:/xampp/htdocs/users/user1/test.zip" "C:/xampp/htdocs/users/user1/test.txt"

Is there a way to use exec() or passthru() together with 7zip when running apache as a service?