Sorry with my wording, I was pointing out Behringer's final item at the end of installing the driver.

I'm not sure about the following, since I don't use Macs very often, but if your Mac allows you to run Linux temporarily from a DVD or USB, you might see if your UMC404HD can run under Linux. I have a couple of Linux DVDs that I use only for hardware troubleshooting on Windows or other Linux distributions since the Linux DVD does not install - unless I say to do an install.

I found the problem. For some weird reason, in the Audio-Midi-Setup the Output 1 volume was turned down completely. I wonder how this could happen.

Quick update to this. I have had a Focusrite Scarlett Solo 3rd Gen here which I was sent to film some video tutorials. I have also had exactly the same issue with my iPad and this interface as I have with my UMC 404HD! This has made me suspicious that there isn't a fault with the UMC 404HD.
Recently I picked up a new iPhone and downloaded Cubasis 3 for it. I tested the Focusrite Scarlett Solo and it worked perfectly so I gave my old UMC404HD a try and guess what it worked perfectly aswell! In fact I didn't even have to use the included power supply like I did when it worked with my iPad mini 4!
It appears that upgrading to IOS 13 on my old iPad mini 4 has done something to the power scheme that has stopped basic audio interfaces from working without having to buy an extra powered USB hub!
I'm super happy that I don't need to replace the UMC404HD as it would cost considerably more to buy an interface that has as many options and also works with an iPhone!

As a result, the Behringer UMC404HD has gone up even higher in my opinion and whole heartedly recommend it for anyone looking for an affordable/reliable and good sounding interface.