When using a third-party script library, I can no longer save an AppleScript file in Script Editor.app
First thing I would try is opening up the scripteditor.app library panel. 
In Scripteditor go to menu item "Window/Library"



If dialog toolkit is present in the list, I would select it in the list and remove it.  Then I would quit scripteditor.app and relaunch it again. Once scripteditor.app is up and running again, Open the Library panel and click on the “Add” button in the menu bar and navigate to ~/Library/Script Libraries and select Dialog Toolkit.scptd.

That will probably solve your problem.  I have a funny feeling the problem is occurring because your new com.apple.ScriptEditor2.plist file probably does not contain any reference to the Dialog Toolkit.scptd as per this example in this image..

This is com.apple.ScriptEditor2.plist file opened in xcode