I can attest to this same issue, and it is the upgrade. Microsoft sent out a patch for it, ideally, after the initial push because it broke FSX, FSX-SE, and other DX9/DX10 32-bit programs.(Evidently it was a known issue and the patch was done, but didn't ship on release day.) The patch came out for it in mid-November.
At first the patch seemed to work, but the problem returned. It is more prevalent with payware on FSX, like the Phenom 300 that I used to love to fly. My system is an 8 core 5960X processor with 32 Gig of Ram and an Nvidia GTX 970.
I considered it could be an Nvidia issue and tried older drivers, but still no joy. I also tried it with DX9 and DX10, as well as the DX10 add-on patch, and without it. The problem is the same. It will run for a little while and then fail when you try to spawn a child window. Easy to recreate.
I'm a Microsoft alumni. I worked there in the 90's, and frankly it's embarrassing to see what they've done with the company in regards to QA. I should say the lack of QA since they laid off most of the department a few years ago. If a Windows Insider doesn't find a problem during "beta", good luck.
There is the hope that they will release a patch for another legacy issue that will fix this, but don't hold your breath. If you have Windows 7, and FSX is that important, I would stick with it until 2020. That said, I think I would yell more at Dovetail at this point since they own the code rights. Unfortunately, they are busing trying to create FSW from the FSX foundation, so I don't think you will hear very much back.