HELP! Can't get my rig to run with more than 3 GPUs

Hello Guys,

I build a rig with the following Hardware:

CPU:      Intel Celeron G3930
Motherboard:   Gigabyte H110-D3A
RAM:      8GB Crucial Ballistix Sport LT DDR4-2400
SSD:      Kingston 120GB SSD
GPU:      6x Sapphire Radeon RX Vega 64
GPU Riser:   Kolink PCI-E 1x to 16x Riser USB 3.0
PSU:      be quiet! Dark Power Pro 1200W

I use Windows 10 Pro 64-Bit and have Problems to get it running with more than 3 GPUs.

- If I have CSM Enable in the BIOS i can Boot into Windows with more than 3 GPUs plugged into the Motherboard
- If CSM is disabled I don't even get a BIOS post, just a the rig running and a black screen

The Problem only occurs when I have the driver installed. So if I plug in all six GPUs wihtout the driver installed i see all of them listed in the device manager as a display adapter (unrecognized)

- If I try installing the driver with the AMD driver package, the rig freezes during the installation process.
- If I try to install the driver manually through the device manager, I can see how he installs and recognizes the 1st, 2nd and 3rd GPU after the 3rd is installed the "installation-progress-bar" and the whole rig freezes.
- If I install the Driver with only 1-3 GPU plugged into the Motherboard it installs the drivers smoothly without any problems or errors. If I try to add the 4th GPU afterwards and Boot into Windows the system freezes in the login screen every time until I unplug the 4th GPU again.

Here are the BIOS settings I've tried:
- BIOS Versions: F22, F23, F24 and F25
- Rig running on internal graphics (iGPU)
- CSM Enabled -> system boots with more than 3 cards into Windows
- CSM Disabled -> system doesn't post (system is running with only a blackscreen no post at all)
- "Mining Mode" Enabled / Disabled -> makes no difference
- "Above 4G Decoding" Enabled
- I tried enabling "Above 4g Decoding" after I installed 3 GPUs before installing the 4th GPU
- "Max Link Speed" Gen1 / Gen2 -> makes no difference
- Disabled "Onboard Sound Chip" and "Serial / Parallel Ports"

Here are the the things I've done and already tried in Windows:

- Installing Windows 10 Pro 64-Bit Anniversary Update and Creators Fall Update (created setup USB-Stick with Rufus)
- Installed Windows 10 in UEFI Mode -> with GPT partition
- Setting virtual memory to 60GB
- Disabled "Windows Update Service"
- Disabled "fast startup" in the power options
- Using AMDs Blockchain drivers
- Using AMDs normal drivers
- Using the AMD Vega BIOS Signature Check Bypass Patch

I'm glad about any help that i get