I have been using QBs for years. All of a sudden data input by different users has stopped syncing. Both users can use QBs independently and simultaneously without any errors. However, the new changes are not shared. We are using a host computer as the server. I have run everything in the Tool Hub.
Then, you can set up your QBDT to a multi-user mode. Make sure that you use to sign in to your server computer has Windows admin rights and the Windows user account for Database Server Manager has admin rights.