Gridelements got the new properties in EXT:start version 5.3.0:
CSS classes for columns
Same height for all columns
If you like to use the new features, you must update the XML-code for the gridelements flexforms.
Look for the folder, where the layouts of the gridelements are stored. Usually a folder named "Layouts" in your page tree.
Copy the code from below into the field "Content Element Configuration: Flexform Configuration" at the tab [Configuration].
Sorry for the trouble.