C3422WE, two PC inputs DELL, MacBook Pro, USB not recognized

Hi all,

I have a Dell notebook and a MacBook Pro connected to C3422WE. Dell notebook is connected to the monitor through USB-C and MacBook Pro is connected through HDMI and USB-B (with the help of Apple's USB-C digital AV adapter). 

Everything works fine. The only problem is that when I change the input source from Dell to Mac, I receive "USB not recognized" repeatedly in Dell (just the sound since I changed the input source of the monitor to Mac already). If I disconnect the USB-B from Mac I don't get the notifications anymore.

Any help to resolve this is appreciated.

Cheers