I am amazed that none of the solutions from Intuit admins included the suggestion to use the Quickbooks Database Server Manager as a solution.

I had a persistent problem with -6190, -816 errors from one of my client computers after a fresh update of Quickbooks Enterprise 2023.  None of the typical spammy fixes worked. I say spammy, because the renaming of the TLD, and ND files to OLD fixes are repeated on many spam websites with link bait about -6190 and -816 errors.

The Quickbooks Tool Hub is also a lame utility that tends to redirect users to a third party support company.  It is a legit Intuit application, but it is meant to redirect users with problems to a subcontracted third party.

In any case, the Quickbooks Database Server Manager includes a diagnostic that you can run that will find any problems with connectivity between client computers and the computer with the company file including firewalls, virus protection, etc. In my case the QDSM fixed whatever issue was causing the client computer to have problems. This connectivity issue is hard to recognize since the client computer would connect in single user mode but not multi-user and it too several regression tests of sequencing connections, etc. until I got a message on the client computer to use QDSM.
QDSM is located on the Windows Start Menu.  Highly recommended to try it first for fixing -6190 and -816 errors.