Hi Nostromo86,
I can see that you've already done a lot of work to try and get QuickBooks to work in multi-user mode. It's clear that you've put a lot of effort into trying to fix the problem, and that's very much appreciated. Please back up your company files before any additional steps.
I want to add that you can also temporarily disable any third-party firewall or antivirus software that you have running on either the server or the client computer. If this still doesn't help, you may need to add specific firewall exceptions to allow QuickBooks to communicate with the server. The exact steps for doing this will vary depending on your specific firewall software, but in general, you will need to add exceptions for the following: TCP ports 8019, 56726, and 55378-55382.
Refer to Step 3 in this article: Set up firewall and security settings for QuickBooks Desktop. However, I highly recommend having an IT Professional do a thorough setup since we can only provide the basic steps.
If you are still having the same issue, it will be helpful to contact QuickBooks Desktop Support for further assistance. They will be able to provide more specific steps based on your setup and configuration.
You can also go back to this thread for any additional questions you may have.
