Search Engine Robot Simulator giving error on page

[st3v3]

To check my site I used the Search Engine Robot Simulator on my site. The homepage URL gives the page and URLs correct, but when I click on a page 'under' the webroot, the Search Engine Robot Simulator gives the following error;

SE Bot Simulation Results
[url]
There was an error while retrieving the URL specified

When accessing the page in a browser, all looks fine (HTTP header '200 OK')... Furthermore, all pages are XHTML1.0Strict valid.

What could be causing this error/alert??

[XML-Sitemaps Support]

Hello,

what is an example URL?

[st3v3]

The site is:
[ External links are visible to forum administrators only ]
One of the URLs that causes an error, is:
[ External links are visible to forum administrators only ]

[XML-Sitemaps Support]

Please check now.

[XML-Sitemaps Support]

Actually, it looks like your site blocks all requests made with googlebot user-agent, as a restult Google cannot index your pages.
Compare this request done as "XML Sitemaps" user agent (HTTP Code 200 returned): https://www.xml-sitemaps.com/http-headers-viewer.html?op=http-headers-viewer&go=1&pageurl=http%3A%2F%2Fwww.humanofficeshop.nl%2Fproducten%2Fkantoormeubelen%2Fdefault.aspx&se=xmlsitemaps&submit=View

This is done as Google Bot (HTTP Code 500 returned): https://www.xml-sitemaps.com/http-headers-viewer.html?op=http-headers-viewer&go=1&pageurl=http%3A%2F%2Fwww.humanofficeshop.nl%2Fproducten%2Fkantoormeubelen%2Fdefault.aspx&se=googlebot&submit=View

You might want to address this issue (in your server configuration) asap.