Good thought/effort superpie.
@ everyone having troubles:

Because you place a pk3 in your base folder, you now *do* have invalid pk3s (duh). If you want to use this, try playing on SST or similar non-pure servers. To fix the error for other servers, simply remove z_superpie_collab.pk3 from your base folder. You don't have to delete it, you can simply move it.