Mobile site of twentyfourteen theme not working (error 404)

Hi there,

I have a strange little problem with one of my pages.
The page is perfectly behaving on a desktop but gives me a 404 error on my cellphone (iPhone 5).
I have tried it on different mobile browsers such as chrome etc. and I still had no success.
Only after deleting all my history and cookies the page was working like a charm for like a minute and then I had again the same 404 error.
Might that be a caching problem (although I have empties the cache of the wp-caching plugin)?
The html is: hi-lessons.com/student/

I would appreciate any help on this.

Thanks guys