I have currently the issue that i cannot save / mark a listing as favorite.

I hope someone is able to give me some explanation or help me.

Usually it means you have some PHP error in footer and scripts required by Favorite items plugin are not printed after footer (invisible).

This is the url I have for a test test offer I made: look at screenshot for the url

Their were some undefined variables inside the footer I corrected them. But now when I Click on the make favorite button (star) I got the error that the ajax url is not defined or that it has the wrong path but i did change it. I tested it on different browsers also in incognito mode.

This is the url it gaves me when clicking the favorite item star: look at screenshot for the url in other post
But I did change it to what I will show inside the screenshot in the other post

This is the error i got in the console

if there were errors in footer, these must be first fixed.

This was indeed the solution, thankyou!