The property for Creating campaign is a bit different than the one used for the LPO that is malfunctioning. They both have page-picker logic though, but this only works for the campaign prop (regardless of language).
I am not able to debug this in firebug since this is an ajax-postback that is called from a compressed js-file, calling a method in a built in assembly (at least no success so far).
Another possible factor: The site in question is originally a migrated epi4-site, but all assemblies are present (done a complete compare of bin-folder), Web.config is also equal to the extent possible, and the episerver core files I expect to be handled by the installer...). The best idea so far is that it may be a built in script that is written for an older version of EPi, or something like that.
Otherwise still quite stuck :-)