If you see the "Content Missing' message within Kontakt, this means it's unable to locate the necessary samples for the library you are attempting to load. If you have missing files, it will look something like this:
Performing a Batch re-save will correct the file paths of the samples for all KONTAKT Instruments within a Library.
Note! In case you need to open the Library with an older KONTAKT version, use this older version of KONTAKT for the Batch re-save process. If you batch re-save a Library with e.g. KONTAKT 7, it will no longer load in KONTAKT 6, KONTAKT 5, etc.
Resolution
Navigate to the File drop-down menu within Kontakt by clicking on the Disk icon and select Batch re-save.
A warning dialog appears. Click Yes to proceed.
Locate the Library folder in macOS Finder or Windows File Explorer. Once selected, click Open to proceed.
At this point, Kontakt will check the Library. If samples are missing, a Samples Missing dialog will appear. In this dialog, click Browse for folder.
In the Locate Folder window which now appears, select your library's Samples folder. Once selected, click Open to proceed.
Kontakt will now batch re-save the library and re-link all the library's instruments to their respective sample content.