more quicktag breaks theme layout

I’m sorry if this is a repeat of another thread somewhere, but I tried to find a similar one and had no luck.

When I use the more quicktag in the middle of any of my posts, it does more than just hide the rest of my entry; it also screws up the layout of the entire theme. The sidebar menus (eg. links, blogroll, categories) get shifted down to the bottom of the very last post on the page. This problem seems to apply with other themes, too.

Here is my current website: https://www.goldfishcake.com

Thank you for reading this; I appreciate your time.