webapps: User can't save a time card when he duplicates the TC

One of our employee is able to duplicate timecard but can't save the TC.

Below is the message from web client

7/14/2016 8:26 AM AssignmentGridSave InvalidOperationException Sequence contains no matching element 
   at System.Linq.Enumerable.First[TSource](IEnumerable`1 source, Func`2 predicate)
   at Microsoft.Dynamics.SL.Mobile.ProjectTimecard.ProjectTimecardController._Closure$__60._Lambda$__94()

Any idea how to fix it?

Any help would be appreciated.

** we are currently at SL 2015CU1 version