I never succeeded to start it as daemon without this. You can start it manually instead of as service, e.g. in your autostart folder.
"C:\Program Files (x86)\Repetier-Server\bin\RepetierServer.exe" /c "C:\Program Files (x86)\Repetier-Server\bin\RepetierServer.xml"would start it from command line as current user. Make sure your use can access c:\ProgramData\Repetier\Repetier-Server as files might now be owned by admin.