False alarm and the issue was that the ML-contact on the BLC side had slided out a bit and noticed that the specific unit doesn't hold back the ML contact proper; so after realigned and refixing it the audio works in both channels once again. Assume that the previous owner must have used a bit too much force when pulling out the ML-cable from the BLC. Does anyone know if it's possible to order replacements of the ML-contact on the BLC PCB?