All of a sudden users of QuickBooks for Mac have started experiencing the loss of connectivity when attempting to enable multi-user mode. There doesn't presently seem to be any know workaround for this 'bug' according to QuickBooks Technical Support.
Accordingly the only advice for QuickBooks for Mac 2020 users is that they work in Single-user mode on their Company files until Intuit can resolve this issue.
A recent information update from QuickBooks Technical Support indicates they have identified the cause and also identified a 'fix' which is presently being tested. They hope to be able to pass along a resolution soon.
8/12/2021 Update - Resolution:
This issue has now been resolved by QuickBooks Technical Support.
The resolution involves download the latest R10 updates, to do this you need to open your QuickBooks for Mac 2020 and the proceed to the download of the R10 update by going to the QuickBooks menu, then choosing Check for QuickBooks Updates.
Once the R10 update has been installed you should be able to enable multi-user mode under the File menu. You need to make certain that all QuickBooks for Mac users have downloaded the R10 update and installed it before connecting to the Company file.
For more information about setting-up multi-user mode on QuickBooks for Mac see this article https://community.intuit.com/content/L8pJEwbXd_US_en_US