Can you log in with admin account?

Looks like an upgrade error. Please file a ticket with customer support as you may need a hotfix.

See Versioned ClientLibs YOu can comment at that URL too for questions why its not working.

Great responses!

As Gaurav points out - see this doc topic: Administering Tags Are you seeing behavior that differs from the AEM documentation?

AEM comes OOTB with a JQuery version. Make sure that the JQuery Lib is part of AEM. One way you can test this is to create a custom component with a client lib and see if you can add a clientlib with a property where dependencies is cq.jquery.See this AEM HELPX article - where is use this property. ...

I am happy to hear it is fixed.

WHy are you not using the Maven Archetype as discussed here: Getting Started with AEM Sites Chapter 1 - Project Setup This is what Adobe recommends. 

Can you please try your issue on a fresh AEM Server and see if you can reproduce.

WHen you want to pass given values to a backend Servlet - using AJAX is a standard way. It's event-driven and typically is fired when a user clicks a button on a page - as opposed to when a resource loads in AEM.