I think I have the wrong RAM after all. Especially ordered with ECC. Turns out the DE3815TYKE doesn't support that. I've ordered new ones. I'll post if that solved my problem.
Thank you for contacting the Intel® Communities.
Regarding using ECC RAM in a not-supported system, mostlikely it won't boot for that reason.
The ECC RAM would still try to perform it's ECC procedures but the system wont recognize them which would lead to issues while booting or interacting with the system(RAM/Computer).
Just make sure you are using a compatible set of RAM: DDR3L-1066 1.35V SO-DIMM
Please do provide us with an outcome when the RAM is changed to a compatible module.
New RAM without ECC works. If someone still cares he can put those (F3-1600C9S-4GSL) on the working list. Thanks.
Thank you for providing us with the outcome on this situation.