OK so he have a few clues as to what might be happening.
As part of a large team we have a few environment artists who work on the same environments at the same time. Let say artist A and B is working in 4.24 and 4.25 comes out and artist B updates to version 4.25 and pushes an art asset or streaming level. Artist A comes along and updates his project files, including the updated level, the level will not be visible to the project until they update to 4,25 as well. Since the link data is contained with in the persistent level the reference is still intact, including streaming levels, so would be greyed out or or referenced as a null object.
Check and make sure that your updated to the latest version of the engine including any plugins