Magento 503 error after (re)installation of LiteMage Cache extension

After (re)installation of the LiteMage Cache extension in the Magento Connect manager, the Magento site can't be accessed and gives 503 errors. During the installation, the Magento Connect manager outputs the error message Exception during cache and session cleaning.

When Magento Connect attempts to install any new modules, it first creates a maintenance.flag file in the root directory. The presence of this file signals that the site is now in maintenance mode and changes can be safely made. Under normal circumstances this file should be automatically deleted once the installation has completed, but certain errors can cause Magento Connect to halt before reaching the removal step. If this occurs, LiteMage will still be installed but your Magento site will become stuck in maintenance mode and therefore be inaccessible.

Simply delete the maintenance.flag file from the root folder of the affected Magento site.