I found a "Terra STD/BTO System" (Model 1300065; SN R1342944) lately and (with permission) took it home for testing. I checked all cables and went to starting the server (first time for me working with a server and I have been quite curious. Unfortunatley however the Power-Button would not work at all. I pressed it, pressed and hold; nothing.
I saw a small LED beeing lit and after research (again I am new to servers!) I found its the Status Fault LED.
I tried changing the PS, took the RAM out, cleaned, put them in seperately, on different lanes, tested the Fan, the HDD. I tested if the intruder lock (small button at the back of the case connected to MB) is not working somehow, but it is totally fine. I tried starting with a bridge over the two Power-Switch pins on the MB in case the Front Panel case was broken.
Worth to mention, that (leaking another PS) I used one with a 4-Pin CPU connector instead of a 8-Pin one.
Only thing I got to achieve once was lighting up the Network 1 LED with first connecting the Power-Switch Pins on the MB and then (eventhough it makes no sense) placed the connector bridge on two pins on the MB called "SCSI_HDD". The Status Fault LED was then turned off. This only worked once, but it might help?
After several attemps to even get to the BIOS without luck I took out the connected HDD to see what OS is running. It turned out to be a Windows installation, however I could not find out which one (I can check again when home again).
If there is anything left to try out, that I haven't already please tell me.
I can test any suggestions, but it will take time as I am not home currently...
EDIT: After building out the mainboard this evening I could have a closer look on the pins and it turned out, that the pins CoolerFault were turned on. I checked the cooler installed and it worked well... However when building it back in, I managed to bend that many CPU Pins that I could not fix it... So with the CPU ensured to be broken I checked what it has been and CPU's for that socket are extremely cheap. If it has been something with the CPU that should then fix it. If it actually is something with the fan, I will take an old case fan (3-pin) out and see if it will run with that.
Thanks for any suggestion in the answers. I now do hope that the new processor will fix it.