Reason:
A blank record exists in the InventItemPrice table for the company which you are currently logged in as.  A validation occurs in the InventTableModule\validateField method which checks if a record exists, since it finds a blank record the Unit of Measure value cannot be saved.

Workaround:
Remove the record from the InventItemPrice table which has a blank value for the ItemID field and the DataAreaId match the company you are currently logged in as.

How it possibly could have occurred:
The InventItemPrice table is a common table which imports occur within Dynamics Ax, a blank record may have been imported during the pricing update.