How to reproduce:
1. Make a new web content portlet, add a new article
2. Publish the new article
3. Edit the article, save as draft
4. Click "Back to page" (not back button in browser, but the link on top right)
6. Edit the article
7. Click "Publish" or "Save as draft"
You have entered invalid data. Please try again.
Another user has made changes since you started editing. Please copy your changes and try again.
This is a quite common way of working for our users of the Liferay CMS. The only way to solve it is the delete the draft and copy the content back in. Not very user friendly...