My problem was dat two versions of the Select2 plugin was loaded. The ACF update loaded a newer version than i was loaded myself. For me the trick was to unload the ACF version i was loading my self. Please check the sourcecode of your backend page to see if multiple versions of Select2 are loaded.
Unloading the other version of select2 fixed this for me.
I found out that this has something to do with another version of Select2 thats been loaded by a plugin or whatever.
Someone of the ACF team? This is quite a big bug for me. I can not use the newest version of the plugin in combination with the post object field because of this.