Hi Roel,
Good to see that the UPS shutdown software is working well. In order for the server to automatically restart following a UPS shutdown event you just need to set the server BIOS status to "always on". It sounds like you have it set to "last known state" or similar right now. When you set the BIOS to "always on" the server will sense when the UPS turns back on (because there will be power applied to the server power supply) and it will automatically turn on.
Hope this helps.
Andy