Hi, I know it is a bit late but just in case it helps someone... I had this issue also and it is apparently due to corruption of the inventory classes associated with your device settings. I fixed mine by:
1) Remove hardware inventory from the custom settings in question and save.
2) re-add Hardware inventory to the custom settings.
After this, I was able to Set Classes appropriately again. Unfortunately, I lost any previous customizations but in my case, that was not a huge issue.
Hope this helps someone.
