Thank you for the information, Ingar.
One of the reasons it keeps on switching from multi-user to single-user mode is because of the network setup. Let's download and run the QuickBooks File Doctor. This will fix company files and network issues in QB Desktop. For the detailed steps, please check this article: QuickBooks File Doctor.
Furthermore, we need to make sure that your Database Server Manager is properly set up. It's the tool that allows users to access the company data. Please check this article about QuickBooks Database Server Manager and let's perform all the steps provided.
Let me know in the comments if you need more help.
