Edit Gallery, Add to gallery Images not showing

Hello

I cannot see images when I edit an existing gallery or try to add an image to an existing gallery all I get is the output below where images should be.

<%if(thumbnail){
%><%}%><%if(
uploading){%>
<%} %>

I removed the post completely, created a new one and added a new gallery successfully all images showing and post displayed correctly. However when I go to amend the post gallery the same issue happens again!

<%if(thumbnail){
%><%}%><%if(
uploading){%>
<%} %>

All media is displayed correctly within ‘media’ with the option to edit etc working fine. It only seems to be within posts if I want to update or add to the gallery from existing media.

I am using a child theme, no changes have been made to functions.php regarding image / galleries (media.php).

Hope someone can help point me in the right direction or has an idea what is happening here? Thanks in advance