What are the U2414H 20 digit serial numbers?
What are the U2414H revisions A00, A01, A02, A03?
Can you remove the working U2414H from the setup? Test the problem U2414H by itself on both of the dock DP out ports to mDP in on the monitor.
Do you have a DP to DP cable to test DP in on the problem U2414H?