Since I updated to V6 it happens 1 of every 3 times approximately that postman hangs on the startup screen and nothing happens even after a few minutes of activity.
I was experiencing this with both the standard and canary versions on Windows 10.
From running the .exe from the command line (Windows 10), the cause of the problem on my machine appears to be that Postman was trying to rename a folder in ...\AppData\Local\PostmanCanary and getting an access denied error.
Solution steps (Windows 10):
Kill all running instances of Postman. I did this using handle.exe "postman" and taskkill /f /pid <pid> using the process id of each running instance of Postman.
Edit the folder security to grant the user that Postman runs under “modify” permission on the folder. Use the Advanced button to replace all child permissions with inheritable permissions (checkbox at the bottom of the dialog).
for the windows users, right click on the app icon and select troubleshoot compatibility. follow the instructions and off you go.
Hope this fixes the issue as it did for me.