Hi there,
I see several users have had the same issue and reasons may vary. I have been trying on different browsers to edit my portfolio pages with Cornerstone. I have cleared cache, disabled browser extensions, disabled third party plugins on WordPress, and the issue persists.
My WordPress version is 4.9.1
My X version is 5.2.5 and Cornerstone is 2.1.7.
site: www.marcelomartinezc.com
My current host is shared with a friend for his professional needs so I don’t know the details about the provider and its configuration. I can always reach out to him to change what we need to change, e.g the PHP memory limit.
Hope you can help me with this, thanks in advance!

