Hello!
I updated my Siet from Joomla 3.2.0 to 3.7.3. Most things went fine, but:

After the update the dropdown navigation does not work as before. The links are not visible. There is an javascript error shown:

Uncaught TypeError: $(...).getElements is not a function (1)

    at Object.initialize (dropdown.js:1)

    at Object.e.extend.$owner (mootools-core.js:38)

    at new <anonymous> (mootools-core.js:37)

    at de:277

    at mootools-core.js:121

    at Array.forEach (<anonymous>

    at Function.forEach (mootools-core.js:5)

    at Array.each (mootools-core.js:9)

    at fireEvent (mootools-core.js:120)

    at HTMLDocument.j (mootools-core.js:175)

Link to the old website (Joomla 3.2.0): http://www.actionsystems.joomla-demo.at/index.php/de
Link to the updated version (3.7.4): http://www.actionsystems-neu.joomla-demo.at/index.php/de

Can you please help me to solve this issue?

Thank xou very much in advance!
Have a nice day, Max