problem with "Approval Center"

When I try to open "Appoval Center" to approve timesheets I obtain this error:

"An unhandled exception occurred in Microsoft.Office.Project.Server.BusinessLayer.Statusing.StatusingReadStatusApprovalsForProjectForGridJson......"

If I go to "Server Settings" -> "Timesheet Adjustment" I can see all timesheet, open them and approve them if needed (after that I obtain the same above error )

someone can help me on this??

thanks

Daniel