I have found a solution for this problem. Just don't waste any time in trying to edit the source code to fix the problem like I did. Just follow the next steps:
Go to apperance > menus
Add a new menu, call it something like "Top menu". Then select the pages you want to add to this menu in the left side bar. Click on add pages when you have checkt the boxes of the pages you would like to add.
Now you will see a row of pages in the right side bar. By dragging them a little bit to the right, you can create a sub menu. Now you can also create a sub menu within a sub menu. When you're done, just click on save changes (or something like that). That's it.
What this does is overwriting the existing menu of your theme. The 2nd submenu will now roll out as it should do, instead of rolling up or whatsoever it was doing before.
Annoying problem which doesn't need a code-based solution, thanks WordPress! :)