10.3RC2 installation is not booting after connecting sata drives anymore: waiting for xpt_config

I created a bug report for this but maybe I am simply to blind to see an easy solution:
There you can download screenshots i rendered from screen capture video.

At the end of the day more people see the forum than reading bugreports.. maybe someone knows what to do:

I installed a 10.3 RC2 UFEI encrypted ZFS System on a Supermicro X11SSH-LN4F Board with a PCIe SSD directly connected to M.2 Port. Fast little beast which only boots UEFI but boots smoothly

After having base system up and running I plugged in a 8TB WD RED Disk in a "Mobile Rack Drawer".
The Mainboard was in hotplug mode, OS has seen new device and I configured a ZFS pool successfully...

Unfortunately the machine is not booting anymore. It asks for a password for root ZFS pool but what every I type in it is not reacting anymore.
Something else has locked the machine already and simple counts seconds:

still waiting after 30 seconds for xpt_config
still waiting after 60 seconds for xpt_config
still waiting after 300 seconds for xpt_config

I switched off hotplug mode in bios: not booting
If I remove the drive: system boots normally....
I replaced WD 8TB disk by an older Samsung 320GB drive: not booting
I connected drive to different SATA port without mobile rack: not booting
I removed SATA disk again: booting

As I never cross the point of uncrypting root filesystem I have no dmesg from this... but I created a video with supermicro's IPMI and was not lazy and copied it by hand. This is all suspicious what I found in bootmessages:

acpi: <SUPERM SuperM> on motherboard
ACPI Error: [?_SB_.PCIO.XHC_.RHUB.HS11] Namespace lookup failure, AE_NOT_FOUND (20150515/dswload-219)

ACPI Exception: AE_NOT_FOUND, During name lookup, catalog (20150515/psobect-233)
acpi0: Power Button (fixed)

ahci0: AHCI v1.31 with 8 6Gbps ports, Port Multiplier not supported

I have no port multiplier in place .
I installed 10.3RC2 with the risk that it fails but I was not expecting such a generic thing at boot time. Has anyone an idea what to change in SUPERMICRO's bios settings? there are not much sata options and i alreády tried on/off hot swap, different ports and I won't switch on raid.
It cannot be a deep hardware problem because a hotplugged HD after boot is detected, and used normally by OS.
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