Cause
A part of a project was filed off as a subproject <x>. Subsequently a further subproject was filed off from this filed-off subproject. When the subproject <x> is restored into the original project, it is subsequently deleted. The project filed off from the subproject <x> is retained, but can no longer be restored.
Solution
Restore the subprojects in the reverse order in which they were filed off.
Note:
Using a new check run (any desired scheme) this module-specific message can be deleted from the message management dialog.