Custom Header Image Upload not Working

Developing my own theme using xampp.

Trying to add custom-header to my theme. Everything seems to be working fine, except the images I’m selecting aren’t uploading (getting error) and the media explorer/viewer is not loading. I’m assuming there’s something wrong with my ‘functions.php’ file, but for the life of me I can’t work it out.

Another note: When I remove my code and check the media library, for example, when adding an image to a post. The images which previously had an error in the header upload, are all there.I can’t find the problem solution on the web. Tried disabling plugins and different methods of adding the custom-header to my theme.

Edit: This happens in both Firefox and Chrome.