Jaguar CD - No image at all

[magiq]

I'm trying to repair a Jaguar CD and I'm pretty much out of ideas at this point. This seems like the best place to ask for help
Maybe someone has seen a similar problem before.

I recently got a Jaguar CD addon that unfortunately doesn't work. When connected to a (working) Jaguar, the power LED is on but there's zero image output from the Jaguar.
Same thing if a game cart is inserted to the unit.
Both the case and the mainboard are in really good condition. It doesn't look like it's been opened before.

Things I've tested/done so far:
- Both cart slots are clean, no missing or bent pins. I'm certain the connection is good.
- Can't find any broken or suspicious traces or vias
- There's clean +5V at Orbit/Butch (U9) and the BIOS EEPROM
- Power Reg U5 outputs +7V, U13 +5V (with PSU connected)
- The BIOS EEPROM is socketed on this unit. So I was able to easily check the contents with an EEPROM reader. It matches the BIOS found online.
- Checked for continuity between the BIOS and the cart slot to the Jaguar.

And something I noticed as well which could be related to the problem:
Output Enable (OE/, Pin 24) of the BIOS is always at +5V, with and without a game cart inserted.

Most testing was done with the CD drive disconnected and no Jaguar CD power supply.
I've never owned a Jag CD before, but as far as I know that should get the unit to boot and show the 'plug in power supply' screen.

Any help is greatly appreciated!