copter wrote:And yes, I have like n+x times checked that I have soldered everything properly, Data goes to PD0 and Clock into PD1.
What about Vcc? Does the Micro deliver 5V to the keyboard or did you accidentially connect the 3.3V line (directly next to it, if a quick glance at the schematic told me correctly)? Because - when connected, at least the M2 should light up all 3 LEDs as part of the BAT. Definitely not only 2 of them (that's normally the "bad capacitor symptom").