Thanks for thq quick reply.
The pages are in Dutch and in English
(http://www.theatreandfilmbooks.eu).
Search on author name is part of our own version of Advanced Search.
The number 15620 is rather high but even if it was 99999 that should not be causing this problem.
Putting the URL the same as your URL indeed does not change anything. Have there been any changes in data of the Product Type Book since ZC version 1.3.5 ?