So I did an offline upgrade with an DVD image on an USB stick. I got a warning about the nvidia driver but my card is old, so I took the risk. Now I can’t login. The cursor is there, and the resolution seems right, but the screen is all black.
Were/are you using the nvidia driver?
The you need to uninstall it, and reinstall it from scratch (or try without).
The existing installation from Leap 42.3 won’t work with the newer kernel. (that said, you may try to boot the old kernel in “Advanced Options” in the boot menu)
Or are you talking about the Mesa-dri-nouveau warning?
What “old” graphics card do you have exactly btw?
I guess it’s a KDE issue? What could I do?
Probably rather a video driver/OpenGL issue than KDE.
You could try to login to IceWM for a start (press Ctrl+Alt+Backspace twice to get back to the login screen) and provide the /var/log/Xorg.0.log.
And/or try to press Alt+F2 inside the broken KDE/Plasma session, maybe krunner still works and you should at least be able to run applications (konsole e.g.)
If that doesn’t help either, switch to text mode with “Ctrl+Alt+F1”, login there, and run “sudo update-alternatives --config default-displaymanager” to set a different one, xdm should work in any case (that’s primitive though, but should be enough for further troubleshooting).
Because Leap 15.0 uses a different kernel than 42.3 (4.12 vs. 4.4), which is incompatible.
Actually it is recommended to uninstall the nvidia driver completely before an upgrade therefore, and reinstall it afterwards.
Or at least upgrade with the nvidia repo active (for the new Leap version), then the driver should get reinstalled too. Would need internet access during the upgrade though.
Another possible problem here may have been the switch to using libglvnd in Leap 15, which isn’t supported by the 42.3 driver at all I think.
libglvnd is a wrapper library that allows to have several vendor-specific OpenGL implementations, i.e. Mesa and nvidia’s in this case, installed at the same time. The proper one is then chosen at runtime.
That’s intended to solve the problem that installing the nvidia driver breaks OpenGL support for all other graphics drivers (which is particularly bad on systems with more than one graphics card obviously).