hello tobi, after checking this i found that your site get above yoru 64mb memory limit. OPC at checkout takes usually cca 30 to 50mb, but if you have many 3rd party extensions it's recommended to have at least 128mb memory limit.
to debug this sort of problems (blank screens) i added a special option within opc general tab:
it is very useful to know if your site generates anyblankscreens (even hidden that you do not know of), so i added this option there. especially your webmaster will be glad to know if there are any conflicts or problems if you set his email there.
best regards, stan