Page 404 after completing order

Hi all

A problem has emerged and I have some trouble finding the solution.

When completing an order the redirecting of the page to the “order complete” page, the pages fails and gives a 404-error/page not found.

(The order does not get completed, but the shopping basket is emptied)

Any bright ideas on, what the redirection should go to and what file to debug, to find the reason?