I’m not sure, never seen that before. Things to try:
- Upgrade to latest Matrix 5.4
- Make sure the version of Edit+ is the same as your Matrix version
- Try it on a brand new page (with just one content container)
- Make sure there are no plugins being loaded
- Make sure the installation guide has been followed
- If all else fails, send a support ticket in to Squiz for them to look at the problem closer