Hi @maxdiable
Currently, the post object field is quite basic and will attempt to load all options (posts). This is most likely causing your site to break due to the load on the server.
Please instead use the relationship field which makes better use of less querying. Please not it will return an array, but you can easily find the first post in the returned array.
ACF5 (soon to be released) will make use of ajax in post_object field to fix this issue
Thanks
E
