Hi, yes, I checked the gound by adding a ground wire manuall to see if that was the issue but the problem persisted.
I have digilights installed, not a bulb anymore (original bulb socket is till hanging there and I put a bulb there to check, but no change either). The wiring on both of the digilights is soldered to the same lines, I checked that and resoldered them even, including ground (the digilights have a separate, dedicated line that is the same signal for both sides), just to make sure all connections had good connection.
The running light works fine, as does also the turn signal in the passenger side. Only when I push the brake it does not light up. On the other side all works fine (running + brake + turn)