Description of my issue:
I use Frontend backup and works fine in several sites, running Joomla! 2.5.11, and Akeeba Backup Professional 3.7.7 (2013-05-11).
One site I updated to Joomla! 2.5.18 and Akeeba Backup Professional 3.9.2 (2013-12-27). For this site, Frontend backup fails to complete.
I tested on 2 sites that share the same server. To isolate any cron/php error, I typed the command on my browser and it worked to run backup on the not-updated site but does not work on the updated one, where I get the browser answer: 403 Access denied.
Httpd error log does not show any error. Httpd access log shows the connection request sent by cron from another server was received:
54.232.75.122 - - [08/Mar/2014:22:45:52 -0300] "GET /index.php?option=com_akeeba&view=backup&key=[erasedpass]&profile=1 HTTP/1.1" 200 17 "-" "-"