I have spent MANY hours debugging this problem. My workstation could not open the QBW file in multiuser mode. It worked fine in single user. I checked all of the QB xxxxx rules in the Windows Firewall and found that all of them showed the proper port numbers as per the Port Monitor shown in the QuickBooks Database Server Manager tool. I logged dropped packets on the Windows Firewall Private Profile and found that packets from the workstation were being dropped when I attempted to open the QBW file in multi-user mode. After much trial and error, I learned that if I set "Inbound connections" to "Allow" on the "Private Profile" of the Windows Firewall, multi-user worked fine. I also learned that if I created a firewall rule on the server which allowed all traffic from the workstation to the server, multi-user worked fine. Since I wasn't happy about having to debug what appeared to be a QB problem, I called tech support. At first, they claimed that I would not be supported because my QB Pro 2019 product did not allow them to debug this kind of a problem unless I paid for the support session! I convinced them that it should support me because the problem occurred at initial setup time and was never reliable. So the tech took over my desktop and added two firewall rules, one Inbound and one Outbound, which both allowed all traffic with ports 8019, XXXXX (port shown in the QuickBooks Database Server Manager tool). Multi-user worked! So with the tech still viewing, I disabled the new Outbound rule she created and then showed her that all existing QB xxxx rules had the same ports as she put in her rule. But her rule made the system work. We then looked further at all the QB xxxx inbound rules and the only difference between those rules and her rule was that those rules were only applicable to a particular QB executable where hers was for "All programs". Looks like we found a big bug in the QB software which created all those QB xxxx rules! Took all day.