Dropbox keeps quitting on my Mac Pro

[insch]

After years of successful use Dropbox stopped working on my Mac Pro a few days ago. It keeps trying to launch (I see it in the menu bar) and then immediately quits. I can't manually launch it. I get the error message 'Couldn't Start Dropbox' - see grab. I don't have a Firewall running or anti-virus software. I have seen posts about fixing this in Dropbox permissions but it doesn't stay up long enought to access permissions. I need to urgently resolve this issue since I might have to travel this weekend and need to sync my MacBook Pro with my Mac Pro. Dropbox still works normally on my MacBook Pro.

Any advice grateful received.

Thank you, Phil

[Lusil]

Thanks for letting us know and for providing such detailed troubleshooting, @insch! Awesome work. 

If you have any other questions in the future, don't hesitate to give us a nudge. In the meantime, have a wonderful weekend ahead!