It’s a NAV 2013 bug: the problem is that in Customer Card/Customer List the action running page Customer Ledger Entries has RunPageView property set to SORTING(Customer No.), which is a non existing key in Cust. Ledger Entry table; change it to an existing key (like Customer No.,Posting Date,Currency Code) to restore user’s capabilty to change sorting.
The same problem affect vendors, too.
Hope it helps.