NVIDIA 450.57 - No 4k output @ 60HZ

Graphics / Linux Linux
1

Hello there,
I cannot get to run my monitor in 4k at 60Hz under Linux.

This is my configuration:

	OS: Linux Mint 20 (Ulyana)
	Kernel: 5.4.0-42-generic x86_64 bits
	CPU: Core i7-4790 bits: 64 type: MT MCP arch: Haswell 
	Graphics: NVIDIA GM206 [GeForce GTX 950] Driver 450.57 
	Monitor: LG Electronics 27UL850-W

Under Windows 10 I get 3840x2160@60Hz without any problems

Under Linux I can set the screen to 3840x2160 but only at 30Hz.
If I choose 60Hz the screen stays black.

But the output of xandr tells, that is should be possible

DVI-I-1 connected primary 3840x2160+0+0 (normal left inverted right x axis y axis) 600mm x 340mm
   3840x2160     60.00 +  59.94    50.00    30.00*   29.97    25.00    23.98    23.98  
   2560x1440     59.95  
   1920x1080     60.00    59.94    50.00    29.97    23.98  
   1600x900      60.00  
   1280x1024     60.02  
   1280x800      59.81  
   1280x720      60.00    59.94  
   1152x864      60.00  
   1024x768      60.00  
   800x600       60.32  
   720x576       50.00  
   720x480       59.94  
   640x480       59.94    59.93

Any ideas?
Thanks in advance for any help!
Hartmut