The problem occurs because the RadGrid is always reporting null as old values per default. This leads to a comparison from null to "" which is false and generates the exception. This has been fixed now.
To get a working application immediately you should specify ConvertEmptyStringToNull=false
in all your editable grid columns.
<telerik:GridBoundColumn DataField="Name" HeaderText="Name"
SortExpression="Name" UniqueName="Name" ConvertEmptyStringToNull=false>
Now the OpenAccessDatasource always gets a "" instead of null. You should do this setting related to your database column definition. Maybe a "" is what you want by default.
All the best,
the Telerik team