I just re-verified the work around again on my system so there must be some crucial difference in our two systems.
First let us make sure we changed the same things:
In file "... \wp-content\plugins\types\embedded\includes\fields-post.php" line 905 was changed from:
$post_type = get_post_type($post);
$post_type = get_post_type($post->ID);
After this change are you getting exactly the same error "Post not updated. Relationship data missing." or a new one.
My WordPress version is 3.5 and Types version is 22.214.171.124.
Are you running any other plugins? I run only one other plugin BackWPup. (I don't actually use Types for real work but I have it around for evaluation purposes only - hence the stripped environment.)
This error is very strange. The developers states 126.96.36.199 is verified on WP 3.5.0 so this must be working on the developers' system. But it is just completely broken on my system. The above two calls to get_post_type() should be equivalent but on my system at least they are not.
Anyway I find this problem interesting and would greatly appreciate getting more info from you.