Some users have discovered a bug in WordPress 4.2 where a site hosted as a sub-domain cannot update plugins without deactivating them first. They've also noticed that, when Divi Booster is deactivated, it is not possible to update it (the option to do so goes away).
WordPress provides an update mechanism for free plugins in the WordPress plugin repository, which continues to work even if a plugin is deactivated. It does not allow premium plugins such as Divi Booster to use this mechanism and they must therefore implement their own updating system (as Divi Booster does). However, when a plugin is deactivated, its code does not run at all which means that premium plugins are unable to load the code to perform updates. As a result, this bug prevents Divi Booster's update mechanism from working.
As there is no way for Divi Booster to workaround the issue (that I'm aware of), it will be necessary to perform Divi Booster updates manually until WordPress release a fix for the issue.