There has been an update that was recently released which messes with the "QuickBooks Permissions". This update makes it so a specific machine may not be able to sync with QuickBooks. Below, is how this is easily fixed.
IMPORTANT NOTE - This MUST take place on the Machine that no longer is able to sync with QuickBooks.
The steps listed below MUST be performed entirely on the machine that is no longer able to sync with QuickBooks.
1. Have ALL Users exit QuickBooks on all other workstations.
2. Sign into QuickBooks using the Admin Username and Password (MUST be the admin username signed into QuickBooks or this will not work).
3. In Quickbooks, Along the upper left hand corner click on File -> Switch to Single - User Mode. If the only option you see says "Switch to Multi-User mode" then skip this step as you are already in single user mode.
4. Sign into The Service Program on the same computer (The computer that currently cannot sync with QuickBooks). If you are already signed in just click the "Update QuickBooks" Button along the top.
5. On the QuickBooks Maintenance Menu popup click the button that says "Update The Service Program from QuickBooks" and hit YES on the popup.
6. You will get a popup from QuickBooks that asks for Permissions to be set to allow a sync. Select the option to Always allow the software to sync from QuickBooks and select the Admin username from the drop down. Then hit Continue.
7. NOTE - Sometimes there is an additional popup that you must click YES on before QuickBooks allows this process to kick in. You will notice that our software starts to sync with QuickBooks and its going to run a full customer and item sync. As soon as it is done, you will get a popup that says Sync was successful. Click OK on that and you are good to go, the permissions have been reset on that machine.
8. Close out our software on that machine.
9. You can now switch QuickBooks back to Multi-User mode and others can sign back into QuickBooks.
10. Sign back into The Service Program on that machine and syncing will work again.