.htaccess – 500 Internal Server Error

So I helped out a friend with her website about a year ago. She has been doing fine until now that she gets a 500 Internal Server Error every time she posts on the blog.

We’ve contacted the hosting and they say the problem is that our CMS (i.e. WordPress) is messing up the htaccess file. But the htaccess file looks pretty normal for a WordPress installation. This is the content:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

Commenting every line out helps and the site is again visible. But when posting something it is rewritten and the error comes up.

This is the website: https://www.elhornodelucia.es

Have you had similar experiences? What was really the problem?

Thank you!