According to the error message this does not seem to apply, but from your description of the objects and their behavior maybe this has something to do with it?
Found this in the AC 14 Reference Guide (page 349 US version):
Locked elements’ selection dots appear in faded form.
Elements are locked (and display “locked” selection dots) in four cases:
• The element is placed on a locked Layer.
• The element has been explicitly locked with the Edit > Locking > Lock command.
• The elements belong to a Hotlinked Module or an XREF.
In all cases, locked elements can be selected, but they cannot be edited. (You will be informed of
this if you try to edit a locked element.) You can open the settings dialog boxes of these elements,
but you cannot edit their parameters.