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Welcome aboard to the QuickBooks Community, niagarasys.

Multi-user mode support access for two different company files with two licensed users at the same time. Also, it allows three licensed users access one company file at the same time. 

Since the issue persists after following the steps shared in this thread, I highly suggest contacting our Support team. They have tools such as screen-sharing (remote access) that can pull up your account in a secure environment and check the cause of this error. They can also perform other troubleshooting steps if necessary.

Here's how to reach them:

1. In QuickBooks Desktop (QBDT), go to the Help menu at the top.
2. Select QuickBooks Desktop Help.
3. Click Contact Us.

For future references, you can check on these articles: 

• Features Available And Tasks You Can Perform in Single or Multi-user Mode.
• Host Your Company Data File in Multi-user Mode.

Feel free to get back to the Community team if you have more questions. Have a good one!