do you remember if you have done any changes to your site?
which files have you edited lately?
for example, compare header.php of your (child) theme with the original from https://themes.svn.wordpress.org/catch-box/3.4.2/header.php
for more theme specific help, try to post in your theme’s forum at https://wordpress.org/support/theme/catch-box/
Thank you for your reply alchymyth!
I actually haven’t done any changes lately, but I did run a theme update yesterday morning. Everything seemed fine after that…
But I just found some comments on the catchbox site talking about problems with the theme update. Strange that the problems didn’t occur right after the update, but thanks a lot for pointing me in that direction!