Rock PI 4B Rev 1.3 No UART / No Red Led


I’ve got a Rock Pi 4B Rev 1.3 that does not boot. When supplying power using a QC3.0 power supply, the green LED lights up, but unlike in working condition where the red LED lights up when booting, it does not.

I’ve put the same SD-Card into another Rock Pi 4B and the red led lights up.
I’ve tried powering directly via the GPIO 5V rail and the PSU reports 0.8A being pulled. The RK3399 heats up very aggressively.
There is no output on the UART-Pins (determined using an Analog Discovery 2 USB oscilloscope.
When powering using the QC3.0 adapter, I can measure 9.0V at the input and 5V at the 5V pins, so power regulation seems alright.

There is no eMMC module installed, just the SD card. There is no UART regardless if the SD card is connected or not.

Are there other steps to troubleshoot or is this just a dead board?