Comment # 9 on bug 1231227 from Stefan Dirsch

(In reply to hui from comment #7)
> The journal also complains about a API mismatch. But can't find out where
> this is coming from.
> 
> NVRM: API mismatch: the client has the version 550.120, but
> NVRM: this kernel module has the version 550.107.02.  Please
> NVRM: make sure that this kernel module and all NVIDIA driver
> NVRM: components have the same version.

All your nvidia packages are version 550.120. Maybe you're running an older
kernel  with an older driver still. Check your modules

  $ find /lib/modules | grep nvidia.ko

and remove the old stuff.