Resource limits?

I’ve found that this plugin seems to have a resource limit. I have a number of custom post types, each with varying numbers of posts stored.

For a CPT with 77 posts, it works fine
For a CPT with 130 posts, it works fine
For a CPT with 260 posts, it works fine
For a CPT with 529 posts, it gives a 500 error when trying to visit the re-order page (though it will happily re-order posts on the main admin page for that CPT)

The site is running PHP 5.6 and memory limit is 2048M. Max post size is 8M and max input vars are 1000. The site runs WPML. All CPTs are non-hierarchical.

Any tips?