I figured out the problem and QuickBooks support should add it to the page about Error 6189.
If you keep the company file in any cloud folder such as Dropbox, OneDrive, Google Drive or iCloud Drive, you may get "file sharing Error 6189" from time to time when you try to open it. Move the company file to your C drive in a folder that is not part of any Cloud folders. I did that and Error 6189 has not reappeared.
This is exactly right! Intuit should know this and mention the problem prominently. It appears any kind of cloud sync setup can interfere with the sharing logic within QuickBooks. This is not hard to find and even if you can't fix it, you should let users know about it. Please!