That's interesting. Most of the plugins I use come from rosali's bundle which I thought was updated for 0.5.2
Just as a test, I've commented out the plugin section of the main.inc.php file so no custom plugins are loaded and I'm still getting the following errors:
upload_attachment -> attachment_upload
save_attachment -> attachment_save
remove_attachment -> attachment_delete
display_attachment -> attachment_display
get_attachment -> attachment_get
cleanup_attachments -> attachments_cleanup
kill_session -> session_destroy
But none of this explains why they work when accessed on the local LAN, but don't when accessed externally.