Hi Jorn,
Have you powered the board with 5v regulated power from the PDB? I have connected this to an unused positive and ground motor pad. If you haven’t done this then it may explain why the quad won’t arm.
For the OSD, is it defiantly configured for the same lipo that you are using? It could be saying low bolts because it is configured to display low volts for a 4s and you may be using a 3s which at full capacity is below the low voltage threshold for a 4s.
Also, have you configured the FC so that you get telemetry to you transmitter? If so, do you telemetry when the battery is connected?
Tim