Just out of curiosity.. are you double-clicked inside any cell? Have you tried just selecting another cell which is not actively in the "edit" state?
I don't know if that could be the issue.
But as @farazhusain said, a new version of Report Builder was released today, so you still will likely want to upgrade the version to avoid the bug that was recently fixed.
A new version of the Report Builder version 5.6.57 has been released today where issues related to cell locking were reported. If you're using an older one, can you try updating to this particular version and see if that helps?