Complex problem caused by NextGen Gallery

As described in my previous forum post (https://www.ads-software.com/support/topic/images-opening-on-new-blank-page-iso-thickbox?replies=4) I have a problem with the NextGen Gallery plugin not working on my website.

After manually adding the NextGen css in my theme, and by adding

function yourtheme_run_scripts() {
wp_enqueue_script('thickbox');
}
add_action('init', 'yourtheme_run_scripts');

to functions.php (as described here: https://wpquestions.com/question/showChrono/id/1869) I finaly got NextGen running. However this code, added to functions.php makes my other .js files perform weird.

The code in functions.php causes a strange effect on my easing.js (which I use for smoothscroll). Whenever I have the code in functions.php plus when the page calls easing.js, the other .js files stop working. When I delete either the code from functions.php (which will makes the NExtGen plugin crash again) or easing.js from the page, everything works again.

What is causing everything to mess up here? I know the best solution would be that NextGen started working properly, me not having to put in the code manually, but somehow I cant get this to work.

Looking forward to the solution!:D