See:

• https://twitter.com/JadedcatWylde/status/567929492133560321
• http://pastebin.com/XMhNfQFX

This is caused by the GUI being opened client-side, which shouldn't be done. All calls to player.openGui should be wrapped in a !world.isRemote conditional.

https://github.com/tattyseal/CompactStorage/search?utf8=%E2%9C%93&q=openGui