Hi Claus,
If you have issues with an external display connected to Surface Dock, try to clear the display cache.
- Undock your Surface from the Surface Dock.
- Download the Surface Dock registry file. Choose to Open the file.
- Select and run surface dock registry.reg.
- Select Yes to allow changes to your Surface, select Yes to confirm, and then select OK to close.
- Restart your Surface, reconnect it to the Surface Dock, and try your external display again.
Reset the dock and monitor
- Remove the Surface Connector from your Surface, turn the connector over, and plug it in again. Make sure that the connection is secure and the indicator light is on.
- If you’re still having problems, try resetting the power by unplugging your Surface Dock from the power outlet and plugging it in again.
- Try unplugging your monitor from the power outlet and plugging it back in.
Let us know if you need further assistance.