[Solved] 2 computers 1 monitor problem

Hello,

I seem to have a problem, and I'm hoping for some input from you guys so that I can hopefully solve it and make it work again. I'm quite computer savvy, but I'm going out of my mind trying to fix this thing for the past 2 days.

A little back story. If you're not interested in it, just skip to the configurations and setup.
My computers aren't latest or greatest, but they work well for what I need them to do. I use one for my every day works (graphic design, website coding, and making music, among other things), and the other is mainly there as backup for websites that I maintain. Anyway, about a month ago, I borrowed that backup computer to my cousin's son since his laptop died and he needed a computer for school (the usual crap - text processing, powerpoint and stuff like that). I took out hard drives out, since he had one, and he took the case there. Fast forward to 3 days ago, I got an email from the hosting company that they are going to migrate the websites to another server. I thought it would be best to have backups before they do just in case something goes wrong (it happened before). So, I took my hard drives to my cousin's place, hooked them up, and backed-up my websites. However, I couldn't wait for Windows updates to finish, being that they have a rather slow internet connection for it to be done quickly, and he wanted me to check his hard drive because he said it behaved a bit weird, so we agreed that I'd take the computer back to my place for a day, finish my updates and check his hard drive, since I had another computer I could hook it up to if I need to and all the tools I needed for me to do it much easier. And that's where the problems started.

>> CONFIGURATIONS <<
PC 1 (work):
CPU: Intel Pentium G3250 @ 3.2 GHz
MB: MSI B85-G41 PC Mate
RAM: 8GB DDR3-1600 Kingston (running @ 1400)
VGA: GeForce GTX550-Ti 1GB (DVI + VGA, connected to monitor with DVI-to-HDMI converter)
OS: Windows 10 Pro x64

PC 2 (database backups):
CPU: AMD Athlon64 X2 4600+ @ 2.4 GHz
MB: Biostar MCP6P M2+ (nForce 430 chipset)
RAM: 3GB DDR2-800
VGA: Integrated GeForce 6150 SE (VGA)
OS: Windows 10 Pro x64

Monitor: LG 22MP55 (1920x1080x60, IPS, VGA+HDMI inputs)


>> PROBLEM <<
The problem is quite simple to describe. When I hooked PC 2 up to the monitor (integrated VGA -> VGA input), the screen was (and still is) just showing "No Signal" even though the computer boots up just fine. That's it.


>> TROUBLESHOOTING <<
What I tried:

1. 
   ■Re-seated all power connectors and RAM modules into their slots - "No Signal".
   ■Disconnected hard drives - "No Signal".
   ■Took out the CMOS/BIOS battery - "No Signal".
   ■Installed an old 8800GTS card (2x DVI, so I tried both DVI->VGA and DVI->HDMI) - "No Signal".
   ■Tried different power supply - "No Signal".
   ■Switched GTX550-Ti to PC2 - WORKED!!!

>> DEDUCTIONS AND NOTES <<

• 
  ■Windows on that computer is working fine. I can access it via remote desktop app from my Android phone. That would remove CPU and RAM as culprits.
  ■Device Manager is showing that everything seems to be working fine.
  ■Both inputs on the monitor are working fine when connected on the GTX550-Ti.
  ■The 8800GTS card is working properly when put in the other computer; indicates that it's not broken.

The only thing I haven't tried is to take it completely apart and put it back together again. But I have a feeling I'm missing something, and possibly something stupid.

In short... Does anyone have any suggestion or advice how to solve this?

(take a look at the diagram)

PS. I am very sorry if I posted in the wrong section. If it's wrong, would someone be kind to move it into an appropriate section? Thanks!