Using iMac as Target Display - Black Screen

Before I state what is happening, it should be noted that Target Display was working for almost 2 years with the exact setup...

I'm trying to use a late-2012 iMac as target display with a 2017 MBP as the source. After logging in on the MBP and connecting both devices with a thunderbolt cable/thunderbolt 3 to thunderbolt 2 adapter, my iMac screen simply goes dark after pressing command-F2.

I've tried multiple solutions from threads and nothing really helped. I tried different thunderbolt ports just in case one doesn't work well, nothing happens still. Clicking on 'detect display' in system preferences - Displays doesn't do anything either. Both computers have their own latest software updates, conditions are pretty good, no idea why it's not working. Tried to use gfxCardStatus on MBP to select certain graphics card, still doesn't work. My MBP is plugged into the wall too.

What I do know is my iMac does go into Target Display mode, because when I exit by pressing command-F2 again there is a brief blue screen then the normal desktop or login screen.

Anyone had similar issue? Any other ways I could solve this problem? Thank you!