Failed to load resource error after migration to new domain

Hey,
I just transfered my website from my testserver to the live server with the help of snapshots and now I get the following error in the console and the arrows in my portfolio are not displayed anymore. usually they have an image as background, but they seem to not have one now… (the theme ist the7)

Failed to load resource: the server responded with a status of 500 (Internal Server Error)
…./css/less/filter-&-pagination.less Failed to load resource: the server responded with a status of 500 (Internal Server Error)

You can see the error on this screenshot: http://bflow-webdesign.com/wp-content/uploads/2015/08/Unbenannt-2.jpg

And the website with the problem is here: http://kroll.ultrabold2.com/krollenergy/project/agria-werke-bk250/

Do you know what the source of this problem could be?