Accordion closes after i click open

The accordion out of a sudden closes each time it is collapsed. Ive deactivated plugins but no change in behavior

I found this tread with a possible solution but i need to understand where the edit.php is located.

To solve this issue you will need to go to the edit.php page and the CSS Code written below will have to be pasted in the Custom CSS section.

.collapse:not(.show) {
display: block;

The page I need help with: [log in to see the link]