Happy you got sorted, that’s strange that you had to replace your pc. I used to use Unreal 4.2 and 4.6 and started playing with 5.6. It started crashing even on brand new blank projects on 5.6.1. I’m not going to test 5.5 because 5.6 worked.
I just installed Unreal Engine and had the same problem. Both versions 5.6.1 and 5.5.4 were crashing on startup. Reading the comments about recent issues with Intel processors, I realized my processor is an Intel i9 13900k (64 GB, RTX 4090). A while ago, I had an issue with it during an Arnold Render update, which was causing rendering to crash. The solution I found at the time was to update my Motherbord BIOS to version F12 (Gigabyte Z790). Well, now I had this problem in Unreal and thought it might be due to the processor. And that's exactly what happened! I was using BIOS version F12 (Jan 2025). I just updated to version F14 (Jun 2025) and voila, problem solved. UE 5.6.1 and 5.5.4 work perfectly. Update your motherboard's BIOS; it might solve your problem.
I’m on the latest bios - I blame Epic because 5.6.0 worked and I assumed 5.6.1 would also work but it did not. Changing requirements on a minor version upgrade and making no statement about it makes me want to write my own game engine