The answer can be found here:
https://groups.google.com/forum/?fromgroups=#!topic/grails-gwt/8hl245I69tM
It’s a bit amusing that the person who started the thread is a friend of mine. Thanks so much Ian! 😉
Long story short:
either downgrade to 1.3.7 version of Grails OR
“comment out the resources plugin from the plugins section of the BuildConfig if you want to omit it from your application.
I was also able to keep the new resources plugin and make it work by adding this to my Config.groovy file:
grails.resources.adhoc.excludes = [‘**/gwt/**’] ” –