500 Sever Errors On Some Devices Only

Website is conceptbaths.com. Since last week, on 3 devices we got 500 sever errors when adding item to the shopping cart, or choosing an option, etc. It seems all operations using POST requests result in errors; On the same devices logging to the backend would fail. Other 5 devices tested are perfectly fine.

error_log didn't show anything regarding this. No source codes have been made prior to this.

Hosting company said this should be a coding issue and not a server problem. Submitted a ticket to CS-CART team but they couldn't replicate the errors on their devices. Any feedback would be appreciated.

This sounds like server limits / PHP limits being triggered. is this on a VPS or Dedicated server where you can modify these values?