What causes a workbook sharing violation?
 

We use an excel 2002 workbook for our time sheets. All of the formula cells
are locked. The sheets are protected and the workbook is protected. The
workbook worked fine from Friday to Tuesday morning. On Tuesday morning no
one could save a change in their timesheet and we all got a message saying
this is because of a workbook sharing violation.

We cannot figure this out. We have not set up the workbook as a shared
workbook.

Please advise as this is our company time sheet record.