Do you know where I can delete the saved settings from the theme?
On the database it's saved under 'frontier_theme' located on 'wp_options' table. You can also just click the "Restore Defaults" button on the Frontier Options page. I don't think this would help you though, since the only thing of importance there is the width values and your widths appear correctly.
I had contact with our provider and they claim that it isn’t the server. But I don’t get the feeling they looked at it very well.
I'm more inclined to believe that this is a server issue. As I mentioned previously, your site displays differently depending on the location of the visitor.
Check your site here: http://tools.pingdom.com/fpt/
Click Settings. You'll see on the thumbnail that the issue appears when the pingdom test server is set to "Netherlands". The site shows correctly if the server is set to "New York". However, if you set the test server to "Texas", the site doesn't load at all which means visitors from that area can't access your site.
Also probably related, your nameservers don't seem to work properly as seen here: https://www.whatsmydns.net/#NS/www.ontstoken-tandvlees.nl