Crash report here http://pastebin.com/ftNNgDLE

Note: not consistent. Does not occur in test world with cheated in obelisk, does at a base.
Possibly due to item in inventory that would not normally be seen in the obelisk's inventory screen?
Similar crash also occurs with Pam's Harvestcraft Market.

It's difficult to figure out which piece of code is at fault here because there is no mention of either mod in the stack trace logs. I suspect a similar crash would occur if Galacticraft was installed with AM2, if it does it means AM2 is not handling extended inventories properly

Just tested the Pam's Harvest market, seems to work just fine for me.