I have the same problem. I just purchased the theme this week, so I have sent a ticket to support.
That’s the reply so far:
“We did have a similar issue previously due to a plugin conflict. The plugin was “Tiny MCE Advanced”
Can I please request you to disable all third party plugins except fusion builder and fusion core and check if the issue still persists
Also, can you please ensure that your page doesn’t have more than 200 revisions”
Actually, I don’t know what is meant by “revisions”, but I will try the point with third party plugins.