Why did the monitor stop seeing the digital signal?

Question

Monitors

The monitor worked through dvi-d with the base system, during the day of testing I simultaneously threw another system unit through d-sub. (2 more monitors were connected to the base machine). At some point, he stopped seeing the signal from the main machine, but through vga it was normal. I checked the cable (connected another monitor), connected it to another system unit, with and without a parallel monitor. The maximum that we managed to get from him - when dvi-d in the menu says that we need a dual smart solution. Resetting the settings (through the d-sub menu works) did not give anything.