The conflict is between SEO Yoast and Google Site Maps. Whenever any plugin is activated, Yoast SEO checks for conflicts between itself and a hardcoded list of plugins (in wordpress-seo/admin/class-plugin-conflict.php). Easy AdSense is not in that list, but site map generating plugins are. Just that the conflict message is triggered when Easy AdSense is re-activated after updating. The same message will show up if you re-activate any other plugin (including Hello Dolly).
After updating the plugin, it may take a minute before WordPress confirms that it has been re-activated. You may have left the window before that message came up, which may be why the plugin was not activated.
I will close this issue as resolved. Please reopen it if you sill have questions about it.
