It is very unfortunate that you have received that message, but it is not true: your contract can be extended on a temporary basis. The Dutch Parliament stresses that it is not the intention of the WWZ to prohibit extensions due to leave of absence.
On the webpage you will find a video (Algemeen Overleg WWZ) and an official document (Zwangere wetenschappers en verlenging contract) by the House of Representatives, both clearly demonstrate that there should be no question that your contract can be extended. You can contact the labor union (such as VAWO) in case the extension is still denied.
Do you come across other problems? Let us know: http://www.postdocnl.com/black-book/