Have you ever experienced QuickBooks failing to switch to multi user mode with error:

H202 - This company file is on another computer, and QuickBooks needs some help connecting

If your configuration is such that QuickBooks Database Server Manager is installed on your file server and QuickBooks itself on another computer, and that file server happens to be a PDC, you might find that on the file server the service QuickBooksDB26 is stopped (where the last two digits will depend on your version of QuickBooks). Trying to start it manually will fail.

Try to stop the service DNS Server first, then start QuickBooksDB26 again, and then restart DNS Server.

There is a UDP port conflict between these two services. You can always set port exclusions so that they do not overlap as DNS is quite greedy in grabbing a large range of ports when it starts up.