After loading flexmonster.js, debugger says: TypeError: window.$.fn is undefined (flexmonster.js:10) as a result of this in a section later the jquery loading function 'flexmonster' is undefined.

Hello Vag,
Thank you for your question. It seems that you haven't included the reference to the jQuery library. The initialization process has changed a little bit from the version 2.3, to know more about it, please look through our migration guide http://www.flexmonster.com/doc/migration-guide/.
Please, let us know if it works for you.
Best regards,
Dmytro.