Really strange on redirection, any help?

We have a modx site that redirects to the pages in modx fine, but when they go to a dynamic page then it works fine on a browser but not on W3C,

EG.
http://www.propertyforinvestmentabroad.co.uk/the-merricks/

but not:
http://validator.w3.org/check?uri=http%3A%2F%2Fwww.propertyforinvestmentabroad.co.uk%2Fthe-merricks%2F&charset=%28detect+automatically%29&doctype=Inline&group=0

Could anyone think of a reason why that would be the case? modx v. 1.0.3,

Chris,

You can download "Html Validator" for Firefox, in this way, you won’t need to open w3c validator anymore. With "Html Validator" you see a icon on firefox for every page you visit which tells you if it does validate or doesn’t validate.

Do you have any bot-blocking code in your .htaccess file?

No nothing out the ordinary in the htaccess file, that is just the one that’s with modx by default,

Any other things to check?

I got 500 errors literally every other time I checked Bob’s Guides at W3C for a while (actually, it may still be happening). Whenever it happened, the very next time it was guaranteed to work fine. I never did figure it out but I did learn to stop caring about it.