U2412M, not detected by second PC

All laptops and the desktop computer have a HDMI port out. I tried the DVI - HDMI cable with all of them. The screen only works with the desktop computer and the DVI - HDMI cable.

The monitor used to work with all computers via this DVI - HDMI cable. It recently stopped being detected by any computer other than my desktop computer.

The newly purchased DisplayPort - HDMI cable was connected to the monitors DisplayPort and laptop HDMI port. This worked with none of the laptops. I have yet to try that cable with the desktop computer.

A PC HDMI out port cannot upconvert to a monitor DisplayPort in port using a straight one direction only DisplayPort to HDMI cable. So not sure what you mean by, "it used to be that it worked without a problem"?

List all of the video out ports on the three laptops.

HDMI 2 DP  - Is this an active cable? If it's not that it's not backwards compatible and will only work if source is DP and output is HDMI you are converting Lower Bandwidth to higher bandwidth, so an active cable will be required, such as this 

Your old cable since it's working fine with your PC and it's strange how all machines stopped working at the same time. If you don't have an alternate cable, may be reinstalling OS on one of the affected machines would help in isolating hardware vs software

I tried the DVI 2 HDMI cable with another Laptop with Ubuntu 18.04. It did not work.

All the machines work with my other Dell monitor (S3220DGF) using a HDMI 2 HDMI cable.

It does not look like an OS problem to me, but rather a monitor problem.