You would need to determine if the issue is library specific or not. I would create a new library then try the same steps to see if you can add the Medical Term List to a new library or not. If a new library works and you are not using the Sync feature with your library then I would suggest using the Recovery Library command on your library.
If you are using Sync then I would suggest starting a new library then use Tools and Sync to transfer the references to a new library once the Sync completes then you could try to add the Medical Term List.
If none of the above resolves the issue then contact Technical Support directly
1-800-336-4474 option 4 then 1 or