When syncing with QuickBooks you may receive the following error messages:
Error recording Item - A related object is already in use, or failed to acquire the lock for this object.
Error recording Item - There was an error adding, modifying or deleting because it is already in use. QuickBooks error message: Unspecified error
First, completely shut down and reboot the machine. Try syncing again.
If that does not work, have everyone log out of QB. Log into QB as Admin in single user mode and All Orders as Admin then try syncing again.
If the above fails, you should do a Verify Data in QB (File -> Utilities -> Verify Data). If errors are found you will need to do Rebuild Data. ***Please keep in mind this process may take a while.