Categories not showing posts

This is our site – https://www.the-odyssey.org/ based on the basic theme. The problem is that I’ve created some categories, Events and Galleries in the menu so that they display as ‘pages’ on the site. The problem is that these ‘pages’ don’t display the posts allocated to those categories. If I go into the backend and save the menu it works again for a short while, but within a few hours I get the ‘nothing found’ message again. I can access the posts from the categories sections in the sidebar.
We host the site ourselves, and have several WordPress sites using different themes running from the same servers without any problems.