There are lots of fixes out there that sort this issue out including changing cookie_domain, changing the Admin users password in PHPMyAdmin or editing Varien.php to comment out the session code. Lately though, with the most recent versions of Magento 1 from around v126.96.36.199, this has become a bit of a problem again. Where old solutions used to work on older versions of Magento, the old solutions are not working anymore.
So, if you’re getting this error logging into your Magento ...
Problem: Service Temporarily Unavailable is showing on every page of my website after upgrade or adding an extension:
Magento 2 maintenance mode file has the .maintenance.flag (with the leading dot) and is located in the var folder. But HTTP 503 code response can have different causes. For example, the cause could be a webserver misconfiguration or file/permissions/ownership errors.
In lots of cases, after installing a new extension and recompiling Magento 2 code, you need to set file and folder permissions again.