I have two pages on my blog, recipes and travels, that I write blog posts for. I would like the page Travel and the page Recipes to show an overview of all posts from that category (similar to the mega menu). How can I achieve this? 

Thank you so much! I did that. Now, how can I remove the word "Category" from the title?
Now it says: Category: Recipes. I'd like to have only "Recipes"
Please post the the code, if necessary. Thank you.

Thank you so much! I did this yesterday (see screenshot from the customizer) but it didn't work. The word "category" is still visible on the recipe and travel category pages. Further, it seems that other customization is also not displaying on the frontend of the website.
Example: Remove "Related" on blog posts like http://misswalters.com/2017/08/18/fig-anise-biscotti-vegan/
I've activated hide related posts box, but it is still active.

It seems there's a problem with the customizer?
Any ideas what's wrong?

Thank you

Thank you so much for your input!

Thanks! I will do that.
Oh, one more question from my side: The customizer (apperance > customize) used to display on the right side the live changes I made to the site. Now it doesn't anymore. It seems to load, but it never fully loads....

Hi, again one more question:

I have trouble with the preview function on the blog post level. When I try to access the preview mode, I get the current error message, see screenshot attached.

Yes, deactivated all.
Now activating plugins one by one

I am not sure a plugin causes the issues. I deactivated all plugins and the mentioned issues still exists...

Yes, when I first set the page up, it worked fine.
But now it doesn't. Apart from installing plugins and jetpack I haven't done anything really.
So, I cannot recall what I did before it stoped working.
I remember at the beginning I had some issues with wordpress because I switched my domain from wordpress.com to wordpress.org. But that shouldn't cause troubles with your code...(I think)
Can I reactivate the plugins?