If you find you are getting strange warping of the Spitfire user interface, or the GUI has disappeared as per the screenshot below:
Go into the library folder in question and in there you will see some files ending in .nkc as per below;
These are cache files created by Kontakt upon using a library and are usually the root cause for GUIs to go awry.
To solve the issue you will need to ensure you close and quit all instances of Kontakt and delete all of the .nkc files you see in the offending library folder. When this has been done, simply load up any of the instrument files again and you should see that the GUI is back to normal, and the cache files have been regenerated.
If you are still running into issues please contact us.