AV Troubleshooting - HDCP Problems, Laptop displays only a green screen to the projector
How to troubleshoot HDCP problems
This is a problem caused by HDCP (High Definition Content Protection). Some Macs always have content protection turned on. Most systems are HDCP compliant, however rooms with lecture capture are more likely to have problems.
Check if the laptop is currently playing any protected content (Likely iTunes or Netflix in a browser), and quit the program. Restarting the computer may also help.
Try a different laptop cable. HDCP signals are only carried over HDMI, DVI, and DisplayPort cables; however, not all cables are HDCP compliant. Engineering IT loanable cables are compliant.