Yosemite Messages Screen Sharing Disabled

I'm trying to screen share on from within the Messages app with another iMessage user. We're both on Yosemite, and we've successfully used Messages's new screen sharing feature many times. One day, we discovered the screen share button was disabled. It has never come back.

Screen sharing is enabled in the Video menu, and we are both on the most recent version of Yosemite. I'm not sure how to look for answers without even an error message to go on.

I just used Yosemite's screen sharing via Messages.

Getting it to work on both sides required me to enable back to my mac in System Settings under iCloud. After restarting the Messages app on both sides those two rectangles became active and screen sharing worked.

I found that to get screen sharing to work, all I had to do is login to my iCloud account via preferences. After that the screen sharing icon was no long grayed out.