Hi @inx,
I think this is caused by ofxLibwebsockets also shipping jsoncpp (like ofxJSON) - as Dan pointed out here https://github.com/labatrockwell/ofxLibwebsockets/issues/27. Try to delete the libs/jsoncpp folder in ofxLibwebsockets. As long as you also use ofxJSON this should work and not cause any trouble.
I should mention this in the readme.
Hope this helps, if not what error message do you get exactly?