Bulk editing posts wasn't working. After deactivating plugins one by one, I deduced that Google Analytics Dashboard was the cause.
So far I've solved the problem by ticking "Disable Post Stats Display" in the plugin settings, but it's a shame to miss out on such a useful feature of the plugin.
These two topics are arguably duplicates:
...but time has passed, I'm using the latest version of Google Analytics Dashboard, and it seems to affect all Bulk Edit operations (not just changing the author)