Problems between XServer and Displaylink Docking Station

[Theresa]

Dear all,

I'm reaching out to you, because for a while I've been experiencing
problems with my docking station Dell D3100 (that is working with the
latest Displaylink driver) and my Linux system.
While I understand that this is usually not a X-Server related problem,
it seems that the root cause is somehow linked to the X-Server. I don't
know how yet.
Of course I have already reached out to the vendor (DisplayLink support)
to verify and troubleshoot the problem on their side, and to rule-out
anything driver related. After a thorough investigation we couldn't find
any error or issue within the displaylink driver and the evdi component
itself.

Let me first explain what my problem is.
Usually I connect my laptop to my docking station and start the boot
process. While it works fine until the login prompt (I'm using SDDM as
my display manager).
Once I login, the screen goes black for a minute or two, until all of
the sudden my wallet-manager pops up. Other than that nothing else in my
desktop environment seems to start, and it also seems to be quite slow.
The next step is then to switch to TTY and then reboot the laptop.
Ultimately, this means that I currently cannot use my docking station at
all. My laptop however works fine stand-alone.

My current setup looks like this:
xorg-server 1.20.8-2

I currently use the following configs in my X11/xorg.conf.d/

Section "Device"
  Identifier "DisplayLink"
  Driver "modesetting"
  Option "PageFlip" "false"
EndSection

It has worked fine and without any issues over the past years, and the
problems started to occur with the version xorg-server 1.20.6.x at some
point.

The support team at Displaylink suggested that it seems to be a problem
somewhere between X-Server and the SDDM and multiple GPUs.
They said,
"what might be problematic is the fact that you are using SDDM display
manager KDE desktop Environment which might have problems with multi-gpu
setups. This configuration is not supported by DisplayLink.
As an advice you can try to replace SDDM with GDM - Gnome Desktop
Manager and KDE with Gnome if SDDM replacement is not enough.
The black screen and “wallet” window is probably an XServer that has
problems, they might come from wrong configuration or from SDDM and KDE
not correctly cooperating with XServer.
There is nothing wrong on DisplayLinkManager or evdi side."

I am happy to provide any specific log files that would help
troubleshoot this problem (Xorg.log or dmesg output or anything else).
If this is not the right place to discuss, I am also happy to create a
bug report on gitlab (I just wasn't sure which project to choose).

Please help me find the root cause,
many thanks in advance,
theresa