Yes, I am very confident this is the solution.
I was posting previously as charlie303 but I now have a new account under charlieMk6.
I was developing under Windows 7 and just hit a break wall like you. But the exact same code, no modifications, worked perfectly under a fresh install of Windows 10. It's annoying I know, Windows 10 is far from perfect but that's life.
I know a fresh, clean install is required too because initially I updated my Windows 7 to Windows 10 and that did not fix the issue. So, I F9 at BIOS prompt booted into the USB stick install media, wiped my partitions, rebuilt the partition and freshly installed Windows 10. Took less than an hour all up though I know you will have other software to install. My original code worked first time. Many resources to create installation media https://duckduckgo.com/?q=create+windows+10+install+media&t=brave&ia=web
As for product keys you can use your WIndows 7 key to Activate Windows 10 - I did. If you search the web there is a command prompt / powershell command that will display this. You can also use Windows 7 drivers for your hardware most of the time. A bit of con in my opinion as Windows 10 is more like Windows 7 Plus rather than a full upgrade. I would of course do a little research of your own to verify what I am saying. Always take a backup. One possible fix to verify what I say would be to spin up a virtual machine with Windows 10 on it and test that way or maybe you have some partition software and could create a dual boot machine?
As for IG's end what did they change? What stack are they running (OS - web server - databases - etc)? Well they are not going to tell the likes of us are they? But I'm guessing if they upgraded the OS of their servers then in the process they upgraded the HTTPS SSL security. HTTPS is that little padlock in the address bar of your browser. Most users just think it is a fixed and done protocol but the encryption methods evolve over time. Microsoft hard code their methods directly into the OS which is why Microsoft code broke because you can't upgrade the protocols in Windows 7. IG must have done this to the DEMO environment in January and after having no major problems (as far as they were concerned) they performed the same update to their LIVE servers last week. Because there has not been an uproar from Windows 10 developers or Linux developers or Java developers (Java uses its own HTTPS SSL stack in its development environment) they assumed all was well. Those who got caught are, a pound to a penny, all running Microsoft development environments on Windows 7. I wouldn't hold your breath expecting much from IG I'm afraid, it's sink or swim time.
I don't know where you are based (I'm in the UK) or if I would get in trouble for handing out my email address but that's my advice. I'm adapting to Windows 10 now, no changes were needed to my code. I'm not wasting your time, good luck. If you do do a fresh install I'm sure we would like to know how it goes.
Charlie