Error on saving timesheet

Hi,

Some users in our Project Server 2010 SP1 environment are getting the following error messasge while saving their timesheets

The clients machines are all XP SP3 machines with IE 7/8. On clicking ok, all the data gets lost. Typically, users are adding multiple number of custom line classification we have created and also giving comments to the column. Its a standard process so I am unable to figure what could be the cause.

Regards

Atul