As the title suggests, I just had this happen to me.
The Core worked as intended, I cut the board and soldered a pin header to it, as well as soldering GND, VDD and PC0 to a pot to patch it up in the software.
When I connect it to my PC via the same micro USB I've used before (which still works for charging and transfering data from my phone) I get no LED lights and the error message "No available USB device found matching PID/VID". When I try to use rescue mode (DFU) I get the error message "No device in DFU mode detected", which suggests it's a hardware issue. Did I cut the board wrong? Attaching a picture to this post.
EDIT: I just tried removing all connected dupont cables from the pin header, so I'm only left with the board (minus the MIDI section) and the micro USB. I still get the same message.
Any help would be much appreciated!