I think Akeeba Backup is part of the problem because it started after changes in the restorepoints software.
When I try to update Joomla versie 2.5.14 to 2.5.17 I get the "internal server error when updating Joomla! "
In the browser I see:
option=com_akeeba&view=backup&autostart=1&returnurl=http%3A%2F%2Fwww.brainy.nl%2Fadministrator%2Findex.php%3Foption%3Dcom_joomlaupdate%26task%3Dupdate.install%26is_backed_up%3D1
The error log is to big to open, I posted a ticket to my hoster for that already.
When I enable show error's within Joomla I just get the same page without errors.
I know the is a chance it's not up to Akeeba, but still it's strange to me it started after the change that the Joomla update button disappeared from your software.
brainy.nl