Johannes,
Thanks for getting in touch and sorry to hear you’re having trouble here, but hopefully I can help. Something is interfering with the plugin’s stylesheets, which is why your calendar is always displaying in responsive mode.
After viewing the source of the page, it appears that your stylesheets are getting combined into one <link> reference using a plugin named ‘scripts-gzip-hack2.’ If the ‘media’ attributes are left off of the plugin’s mobile stylesheets, then this can cause the issue you’re reporting above. Try disabling that plugin and let me know if the issue is resolved.
Thanks! 🙂
-Casey-