Gallery section (dropdown) disappeared from Posts

Hello,

Recently we noticed that when we open any Post the Gallery sections (drop-down) have disappeared where we would normally select the Galleries that we want to display within in the post. It seems this is now causing that page not to render and gives 500 (Internal Server Error).

Here is screenshot of the page when it was working (notice section in red):
http://nimb.ws/Plcft7

Here is current screenshot, where those sections have disappeared:
http://nimb.ws/92uMfn

Any idea on how this can be resolved so our Posts will work again?

Thanks in advance