Question PC shorting in case, but works outside. PSU sparked ?

Jul 29, 2021
Someone I know attempted building a PC, but once they powered it on, their Segotep PSU sparked with smoke. I have been working on this PC since then with a new Corsair RM850x PSU they bought. The problem is the PC will only post outside of the case and I have no idea why. I have built several other PCs, and never had this issue. The standoffs are in the right place, and it runs fine outside of the case.

The most perplexing part of all this is that I was able to get it to post inside the case for a limited time testing connectors one by one. Once I connected an SSD and USB 3.0 it started shorting out again, and once I unplugged them it still kept shorting! It won't run normally until I take everything out again. If anyone can help I would be extremely grateful, I'm feeling quite hopeless with this. The specs are:

Asus Prime Z490-A
no GPU for now
Corsair RM 850x
Viper 16gb 3600 ram
Musetex G05 case