That kernel was released on the 21st Sept. So it could be a kernel issue.

Suggest boot with an earlier kernel to see if things begin to work again.

Press shift and hold immediately on boot to display grub. Choose the
advances options and select an earlier kernel to boot with.

On Sun, 11 Oct 2020, 12:30 Andre Plötze, <email address hidden> wrote:

> ** Description changed:
>
>   I run Ubuntu Budgie 20.04 on my Lenovo ThinkPad T14s with Ryzen 4750U
>   processor together with the "Lenovo ThinkPad USB-C Dock (Gen 2)"
>   (40AS0090EU).
>
>   The docking station is connected via USB-C to the system, supplies it
>   with power via USB Power Delivery and provides USB 3.1 Gen 2 (10 Gbit/s)
>   connectivity (including an Ethernet controller and audio codec running
>   over the USB data path), as well as support for two monitors via
>   "DisplayPort alternate mode" of USB-C, so technically it will configure
>   the four high-speed lanes of USB-C as one lane for SuperSpeed+ RX, one
>   lane for SuperSpeed+ TX, and two lanes for DisplayPort 1.4, the latter
>   of which the docking station will then demultiplex onto two full-size
>   DisplayPort connectors like an MST hub.
>
>   So far, external monitors have worked fine most of the time. However,
> - since a few days, I cannot get the system to detect the external
> + for a couple of days now, I cannot get the system to detect the external
>   monitors connected to my docking station anymore.
>
>   The "BIOS" (EFI) boot screen (with the Lenovo logo) still shows up on
>   the external monitors. However, both the LUKS/dm-crypt pre-boot-
>   authentication screen (since I use full-disk-encryption) and also the
>   desktop after login show up on the internal display, even with the
>   laptop connected to the docking station, so it appears like the kernel
>   no longer detects the monitors connecting via "DisplayPort alternate
>   mode". Yesterday, I could get the monitors to show up by disconnecting
>   the laptop from the docking station and reconnecting it again. Today,
>   not even this appears to work. Sometimes the external monitors are
>   detected (and shown as "enabled") in the display settings of Budgie,
>   sometimes they do not even show up. Even when they show up and are
>   enabled, nothing is displyed on them. The monitors say they do not
>   receive a signal and I should check the connection. Unplugging and
>   replugging the cables from the docking station to the monitors does not
>   change anything.
>
>   Audio, Ethernet and USB runs fine over the docking station, just
>   DisplayPort does not. This is obviously very bad, since I now have to
>   work on the internal display even with the laptop docked.
>
>   I have the "amdgpu.exp_hw_support=1" in my kernel arguments (*) and the
>   Ryzen 4000 GPU appears to be recognized.
>
>   $ lspci -k
>   06:00.0 VGA compatible controller: Advanced Micro Devices, Inc.
> [AMD/ATI] Renoir (rev d1)
>    Subsystem: Lenovo Renoir
>    Kernel driver in use: amdgpu
>    Kernel modules: amdgpu
>   $
>
>   (*) When I installed Ubuntu Budgie 20.04, I needed to add this in order
>   to extend my desktop over the two external monitors. Without this
>   argument, the system only showed a single display in the settings, the
>   same content was displayed on all connected monitors (mirrored) and
>   there was no way to change that.
>
>   I have attached parts of "dmesg" output which I deem relevant.
>
>   $ uname -a
>   Linux thinkpad-t14s 5.4.0-48-generic #52-Ubuntu SMP Thu Sep 10 10:58:49
> UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
>   $
>
> --
> You received this bug notification because you are subscribed to Ubuntu
> Budgie.
> https://bugs.launchpad.net/bugs/1899336
>
> Title:
>   Thinkpad T14s (AMD) external monitors on docking station no longer
>   detected
>
> Status in Ubuntu Budgie:
>   New
>
> Bug description:
>   I run Ubuntu Budgie 20.04 on my Lenovo ThinkPad T14s with Ryzen 4750U
>   processor together with the "Lenovo ThinkPad USB-C Dock (Gen 2)"
>   (40AS0090EU).
>
>   The docking station is connected via USB-C to the system, supplies it
>   with power via USB Power Delivery and provides USB 3.1 Gen 2 (10
>   Gbit/s) connectivity (including an Ethernet controller and audio codec
>   running over the USB data path), as well as support for two monitors
>   via "DisplayPort alternate mode" of USB-C, so technically it will
>   configure the four high-speed lanes of USB-C as one lane for
>   SuperSpeed+ RX, one lane for SuperSpeed+ TX, and two lanes for
>   DisplayPort 1.4, the latter of which the docking station will then
>   demultiplex onto two full-size DisplayPort connectors like an MST hub.
>
>   So far, external monitors have worked fine most of the time. However,
>   for a couple of days now, I cannot get the system to detect the
>   external monitors connected to my docking station anymore.
>
>   The "BIOS" (EFI) boot screen (with the Lenovo logo) still shows up on
>   the external monitors. However, both the LUKS/dm-crypt pre-boot-
>   authentication screen (since I use full-disk-encryption) and also the
>   desktop after login show up on the internal display, even with the
>   laptop connected to the docking station, so it appears like the kernel
>   no longer detects the monitors connecting via "DisplayPort alternate
>   mode". Yesterday, I could get the monitors to show up by disconnecting
>   the laptop from the docking station and reconnecting it again. Today,
>   not even this appears to work. Sometimes the external monitors are
>   detected (and shown as "enabled") in the display settings of Budgie,
>   sometimes they do not even show up. Even when they show up and are
>   enabled, nothing is displyed on them. The monitors say they do not
>   receive a signal and I should check the connection. Unplugging and
>   replugging the cables from the docking station to the monitors does
>   not change anything.
>
>   Audio, Ethernet and USB runs fine over the docking station, just
>   DisplayPort does not. This is obviously very bad, since I now have to
>   work on the internal display even with the laptop docked.
>
>   I have the "amdgpu.exp_hw_support=1" in my kernel arguments (*) and
>   the Ryzen 4000 GPU appears to be recognized.
>
>   $ lspci -k
>   06:00.0 VGA compatible controller: Advanced Micro Devices, Inc.
> [AMD/ATI] Renoir (rev d1)
>    Subsystem: Lenovo Renoir
>    Kernel driver in use: amdgpu
>    Kernel modules: amdgpu
>   $
>
>   (*) When I installed Ubuntu Budgie 20.04, I needed to add this in
>   order to extend my desktop over the two external monitors. Without
>   this argument, the system only showed a single display in the
>   settings, the same content was displayed on all connected monitors
>   (mirrored) and there was no way to change that.
>
>   I have attached parts of "dmesg" output which I deem relevant.
>
>   $ uname -a
>   Linux thinkpad-t14s 5.4.0-48-generic #52-Ubuntu SMP Thu Sep 10 10:58:49
> UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
>   $
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/ubuntubudgie/+bug/1899336/+subscriptions
>