I know this hasn't been an easy process for you and your clients, NickConder. 

Let me make it up to you by making sure your clients can access the file at the same time. 

The error code that you've encountered usually means that the network firewall is interrupting the connection. I'd suggest following the steps below to resolve this.

To start, I'd suggest downloading the QuickBooks Tool Hub. This tool helps fix common errors. Here's how:

1. Close QuickBooks.
2. Download the most recent version (1.5.0.0) of the QuickBooks Tool Hub. 
3. Open the file you downloaded (QuickBooksToolHub.exe).
4. Follow the on-screen steps to install and agree to the terms and conditions.
5. When the install finishes, double-click the icon on your Windows desktop to open the tool hub.

Then, use the QuickBooks File Doctor. This tool runs a mini repair of your server and firewall.

1. From the tool hub, select Company File Issues.
2. Select Run QuickBooks File Doctor. It can take up to one minute for the file doctor to open. 
3. In QuickBooks File Doctor, select your company file from the drop-down menu.
4. Select Check your file (middle option only) and then Continue.
5. Enter your QuickBooks admin password and then select Next.

Once done, I'd recommend downloading and installing the latest release of your version of QuickBooks Desktop. This keeps your software up-to-date to ensure you always have the latest features and fixes.

If the issue persists, I'd suggest following Step 3 in this article: How to Fix "Error: Cannot Communicate with the Company File due to Firewall".

Additionally, I've added this article that'll help you learn about the tasks that can be performed in single or multi-user mode: Features Available and Task you can Do With a Single or More Than One License.

Please keep us posted on your progress in getting this error message resolved, NickConder. It's our priority to ensure your clients can access the file at the same time.