Joomla 3.4.8,
php 5.6.18 (i think)
I removed the "&" and the warning dissapeared. Super! Thanks!!
Eh, this is what I ment with "error". I'm quitte a Joomla rookie (yet)

Great. Some things are deprecated in later version of PHP.
I'm not aware of the error you mentioned. I bought the Template (Joomspirit 110) and downloaded the menu module here from Pixel Point Creative.
I've just installed "jquery easy plugin". Just have to find out how it works.
I see you got it going. Joomla 3 has jquery by default. Most extensions include jquery as well. Jquery easy strips out all by one and reorders so everything works together.
I also want to delete the borders around the menu en change the letterfont.
I changed these items in flyout.css and accordion.css. I don't see these changes on the site.
Could that be due to the error you mentioned??
No. The error was a function thing (PHP) but your issue is a CSS thing.
Add this to the end of flyout.css. Change font to your choice:
ul.flyout-menu .menu-link a { font-family: arial;}
This for the border:
ul.flyout-menu, ul.flyout-menu.vertical ul { border: none;}
If you want to remove divider lines too:
ul.flyout-menu li { border-bottom: none;}
Hope this helps.
Dan