Hi,
I do not have any problem with your extension, but you have incorrect information in update server
http://cdn.akeebabackup.com/updates/atcore.xml
You can not use: <client>site</client>
It should be <client>0</client>
Now upadte is being found in Joomla 2.5.8 and stored in database but it is not being shown in Update manger because it do not meet any installed extension. I had the same problem with my extensions and it helped due to Joomla Docs about update servers.
PS. I have attended to Joomla! Day Poland 2012 with Admin Tools Pro presentation of Nicolas - it was great and now I am happy user of Pro version.
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Piotr Moćko,
Perfect-web.co web developer