Instead of loading page, my site downloads php file

Ever since I re-installed my site, running the latest version of WP and having the Mimbo 3.0 theme activitated, my site is sometimes ‘downloading’ the php file, instead of loading the wanted page.
It usually happens when I am editing in wp-admin, after clicking the ‘update file’ button. After it happens, it takes a while before the site is running again.

Does anyone know what could be the problem?
I am using Firefox on a Mac.

My site is: https://kittyarends.nl