This issue impacts Mac OS users and Windows users.

If you encounter a black screen after starting RGT in MacOS, press CMD+F to switch RGT to window mode, then select your native screen resolution from the options panel (shortcut: O).
If you encounter a black screen after starting RGT in Windows, press Allt+Enter to switch RGT to window mode, then select your native screen resolution from the options panel (shortcut: O)

We are aware that this issue impacts some users and we are currently investigating this, you can help us by sending these files after you encounter this issue to support@roadgrandtours.com
Windows: c:\Users\YOUR_USERNAME\AppData\LocalLow\RoadGrandTours Inc\Road Grand Tours\output_log.txt
Mac: /Users/YOUR_USERNAME/Library/Logs/Unity/Player.log or (~/Library/Logs/Unity/Player.log)