This error can be caused by the currently used Design Form for the Template Editor; to reset the Design Form specifically for the Template Editor screen:
Login to RDB as a user who can edit design forms, and is currently using the problematic design form.
Attempt to open a Stored Document so that the error appears.
OK the error message - the template editor should appear.
Click Design Form and then click Reset.
Click Yes to acknowledge Reset to Defaults.
Close screens back to the main RDB screen.
Attempt to open or add Stored Documents - this should now work as normal.