Hello - You post your question in the Dutch Forum. The English Windows Phone forum can be found here: http://social.microsoft.com/Forums/en-US/category/windowsmobile/
I don't know exactly what is going wrong in your case with ActiveSync. Are you able to uninstall ActiveSync and delete all the folders ActiveSync uses.
ActiveSync also uses folders in C :\Documents and Settings\[USERNAME] \
Search for folders there to completely remove all the ActiveSync Settings.
Please make sure you have a proper backup of your device!
Good luck and should you have any questions I'd recommend you to post them on the English forums. There are more people around to help you solving your problem.