Sorry to hear about the trouble, we'll try our best to help you with it. Please ensure that all the collaborators have the latest version of Adobe XD. If you are already on the latest version, please try the steps shared below:
Sign out of XD and Adobe CC Desktop app.
Close CoreSync and all Adobe related processes from Task Manager/Activity Monitor.
Sign back in on Adobe CC Desktop app and launch Adobe XD from there and check again if it works or not.
If this doesn't help, please share a few more details like:
A screenshot of the About XD info page.
The version of the Operating System.
Is this happening with a specific teammate or you are unable to invite anyone?
Are you connected to a private network or a managed network like a work network?