Hi there, mhemple.
Thank you for visiting the QuickBooks Community. I appreciate you for performing some troubleshooting steps to fix the issue. This isn't the kind of service we want you to experience while using QuickBooks Desktop for Mac 2020. With this, I'll be sharing details about this matter.
The issue about lost connection error when sharing file in QB Mac 2020 was reported recently as an ongoing issue. Our product engineers are now working to fix this as soon as possible.
For now, the single-user mode is all that can be used as a workaround.
I suggest contacting our QuickBooks Support Team, so they can add you to our notification list. This will help our engineers determine the number of affected users. You'll also receive an update through email once this has been resolved.
I also recommend visiting our QuickBooks Help page, so you can browse articles or questions from other users that were answered by QB experts. This will help you earn more knowledge that can help you with your accounting tasks.
Don't hesitate to post again or leave a comment below if you have any other concerns about getting an error while sharing the file in QB Mac. I'm more than willing to help. Have a wonderful day!