Custom Page – Description page not working

Why do I get a part of my description page popping out?

When I switch to Custom Page, a part of my description gets out the description box and can actually be changed with the mouse. I deleted some of the description to see but it was the last part still popping out.

Please advise on what could be the reason. I tested it on IE and Chrome. Same problem.

Thanks,
Arnaud

https://wordpress.org/plugins/ultimate-product-catalogue/