Admin bar javascript error

I’m working on a (not mine) website using wordpress 3.6.1

There is a order blocking javascript error about admin bar

TypeError: this.on is not a function

This, using firebug, is related to

https://www.<domain>.it/wp-includes/js/admin-bar.min.js?ver=3.6.1

I’ve no idea of where to start to debug, because i’m absolutely new to wordpress world.

How can I identify the error and or correct it?

I know that ‘.on’ is available since jQuery 1.7.

Looking at jquery loading in firebug panel, i see 3 different load (one for 1.10.1 and two for 1.5.1). Of course this could be a problem.

I think they recently updated a plugin to a too new version. But how to identify the cause?