Greetings, Klippenberg.
I see that you collaborate on Keynote presentations. The problem is, it's no longer working, and I'm happy to help.
Keynote for Mac: Keynote collaboration overview -- From the minimum requirements, Keynote 8.1 should work.
Minimum system requirements
To view or edit a shared presentation, people you share with need any of the following:
- A Mac with macOS Sierra 10.12 or later and Keynote 8.1 or later installed. They must also be signed in to iCloud and have iCloud Drive turned on.
- An iOS device with iOS 11 or later and Keynote 4.1 or later installed. They must also be signed in to iCloud and have iCloud Drive turned on.
- A Mac or Windows computer with a supported browser.
Anyone with an Android device, or an iOS device without iOS 11 or later and Keynote 4.1 or later, can view but not edit the presentation.
Note: Not all Keynote features are available for a shared presentation. For more information, see the Apple Support article About collaboration for Pages, Numbers, and Keynote.
How to turn on or turn off your Mac -- Since it should be working, restart your MacPro then test the issue.
Let me know how that goes, and take care!