Don't copy the stringutils.xml file in the additional libraries folder!
StringUtils is a standard library !!!
Remove it from the additional libraries folder.
Just check it in the Libraries Manager Tab.
Additional information:
Libraries have two files an xxx.jar and an xxx.xml file.
For additional libraries you must copy both files in the additional libraries folder.
The xxx.b4xlib libraries are different, these have only file, an xxx.b4xlib file, which must be copied in the additional libraries folder.