Just a thought as I've had strange problems with these monitors? Try connecting your monitor to a standard USB 2 port on your Mother Board? as I couldn't get my X-rite Pro 1 Display to work on the USB 3 ports on these monitors and I had some HDD crash while connect to a USB 3 port so I've stopped using them. Hope this helps.

Hello,

are there other suggestions?

Thanks

Be sure that Display Port 1.2 is DISABLED in the OSD (On Screen Display) -Display Settings. Remove the 1.5.3, reboot. Install the 1.5.1 version. Reboot. Retest.

http://www.dell.com/support/drivers/us/en/19/driverdetails?driverid=70JPG

I did as suggested, but I also downloaded the latest version of the i1 Display Pro (from X-Rite) to get the latest drivers, then I installed the customised version by Dell (the installer did not overwrite the X-Rite communication driver, it only installed the additional binary from Dell). Now it works.