Secondary Monitor only works when primary is disconnected


A few days ago my secondary monitor suddenly stopped getting registered and only displayed 'no signal'. I have a GTX-1080 Strix, the primary display is a DELL TFT S2716DG connected via DisplayPort. I had similar issues when I bought this monitor - back then IT wasn't registered, I had to disable the deep-sleep mode for it to work.

The secondary monitor is connected at the DVI-D port of the graphics card. Since it is an older model (LG BenQ G925HDA) it has a VGA input. So I got myself an active DVI-D to VGA Adapter. This adapter probably broke a few days ago (see below)
OS: Win7 Pro x64, BIOS up to date, graphics driver up to date (Nvidia 385.69)
Hardware: 550W power source, MB: Asus P8P67 LE, Intel i5 2500k

What I've tried:
- connected my laptop to the LG-Monitor to see if it works - it does
- unplug the primary monitor - this didn't change anything until I bought another active adapter to see if the one I have is broken -> desktop would show up on the LG Monitor, but signal is lost again when I plug in the primary monitor.
- reinstalled the Nvidia display drivers
- BIOS "graphics" settings changed from PEG to PCI (didn't change anything, will revert back to PEG now)

any suggestions/ help would be much appreciated Happy

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Clarification Request
That's a lot of info but

I didn't read any words about "extending the windows desktop" and such settings. I find a lot of folk have forgotten that area. Can you look into that?

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that was the setting before it stopped working

but now the secondary monitor doesn't show up in the montor-window so there isn't even an option for it

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