I'm using Linux Mint 21.1 on a Lenovo ThinkPad P50.I've plugged in two identical external monitors (Asus PB287 - 4k) via a Lenovo Docking Station.
The first monitor is using Displayport (should support 60Hz), the second Monitor only via HDMI (there is no second DP slot). It should only support 30Hz that way.
During booting, all three monitors (Laptop + 2 external) are working. When entering the disk-encryption password, I see it everywhere.The same on the login-screen: All three monitors are working. The password prompt is only one one monitor (the HDMI-one).
After login however, the HDMI-monitor turns black.I can see all three monitors in the display settings, but I can't enable the third one (HDMI).When I click the enable-toggle-bar and press "Apply", all monitors turn black for a fraction of a second (which is normal), and turn back on the way they where before. The third monitor is immediately disabled again and nothing changed.
dmesg does not produce any messages during that process.
I'm using the nVidia driver (v525.85.05).
I previously used Windows 10 with the exact same hardware setup, and everything worked without issues. This, and the fact that all three monitors are working during booting, show me that the Hardware-side supports this. It should therefore be a software issue.
I have the feeling that this is because the two external displays are the same type. The nVidia settings dialog shows a GUID for one monitor, but not the other (maybe because it's disabled?). Interestingly, it also shows "Signal: Displayport" and "DisplayPort Connection Type: DisplayPort" for both external monitors, although one is connected via HDMI.