As we saw in that older thread, the fault is not with the Dell monitor hardware. This is how the Nvidia driver views our monitor with DP 1.2 enabled. To produce the required U3415W (21:9, 3440x1440, 60Hz), the Nvidia driver is stitching together dual 1720x720 60Hz tiles side by side. Thus, their driver is seeing this dual tile as an HDCP repeater.
* Press the Menu button * Open Display * Highlight DP 1.2 * Change it to Disable * Exit the Menu * Retest
If the Nvidia Control Panel no longer gives you the HDCP repeater error, then we know it was the Nvidia driver mis-reporting the dual tiles as HDCP repeater hardware.
For my notes, send me via Private Message the monitor service tag number.