I have a early 2015 Retina Macbook. I recently got a Dell 1707FP monitor to use as an external display and I have an Aukey CB-C26 hub that provides me with an HDMI port. I'm using an Amazon basics HDMI to VGI cable. When I connect the monitor, the laptop recognizes an external monitor is connected but the monitor is just a blank blue screen. I know the cable works because when I connect using the same cable to my Windows laptop the monitor works fine. I also know my Aukey adapter works when I connect it to my TV using an HDMI cable.
Here's the odd thing. If I restart my Macbook while it's connected to the external display, when the login page appears it appears on the external display. As soon as I put in my password, the display flickers and goes blue while the Macbook screen wakes up. So, the display is getting a usable signal at least some of the time. Any idea what is causing the blue screen on the external display and if there's anything I can do to change it?