The usual symptom is that after installing Akeeba Backup or taking a new backup or on an unsuspected time, your Joomla! 1.6 (or later) site's back-end is completely blank -no menu, no icons, nothing- except Akeeba Backup's backup notification button. Most people think that Akeeba Backup broke their site. After all, why is only the backup icon showing? It's got to be Akeeba Backup, right? WRONG!
In fact, you have one of two issues: A known issue with Joomla!'s session storage, or an ACL issue
Joomla! 1.5 allowed up to 2Gb of session information storage per user. Joomla! 1.6 changed that limit to around 20Kb, that is 100,000 times less session storage space. Unfortunately, this caused a major issue. Under some circumstances, Joomla! itself and its extensions might require more space than the measly 20Kb allocated. This causes the whole session to become corrupt. Part of what is stored in the session storage is the current user's ACL privileges. Since they are blank, nothing is displayed except Akeeba Backup's module which doesn't make use of ACL settings, misleading users into believing that Akeeba Backup broke their sites (it didn't).
The workaround is simple, but it involves editing your database:
Delete your browser's cookies and cache
Using phpMyAdmin -or a similar tool- find your jos_session table. Please note that the jos_ part of the name may be something completely different on your site, e.g. aiwn_. This part is called the "table name prefix" and it's easy to spot: it's the common beginning of all of your Joomla! tables' names.
Empty the contents of the jos_session table. DO NOT DELETE THE TABLE, only its contents.
Retry to login. If the problem was due to this bug you will be able to log in now.
I have submitted a bug report and a patch to the Joomla! bug tracker. As of August 2011 the patch is accepted and will be included in a future release of Joomla!. Others have found other session-related bugs and have submitted patches for them. Apparently, before Joomla! 2.5 is published all of those patches will be included in the Joomla! core and by January 2012 this bug should be a thing of the past (I keep my fingers crossed...)
Joomla! 1.6 and later versions come with a powerful and dangerous ACL (Access Control List) feature. It's easy, without realising it, to totally brick your site by fiddling around with them. You won't be the first or the last to suffer that. In fact, in early August 2011 I helped yet another user who had accidentally bricked his site by toying around with ACL settings and sent him to the Joomla! forum where the top experts in Joomla! ACL managed to help the unlucky fellow (thank you Jen, Sander and Sumendra!). You can see the full forum thread at http://forum.joomla.org/viewtopic.php?f=624&t=646437.
The solution, as you can see on that thread, is rather simple - and involves some database editing as well:
Using phpMyAdmin -or a similar tool- open your jos_viewlevels table. please note that the jos_ part of the name may be something completely different on your site, e.g. aiwn_. This part is called the "table name prefix" and it's easy to spot: it's the common beginning of all of your Joomla! tables' names.
Find the row whose title field reads "Special" and click on the pencil icon on its left
Change the contents of the rules field to read:
If you don't have this row, please note that you deleted the most basic view level of your site! We will have to create it afresh. Create a new row with the following values:
and save it.
Now you should be back in business. And, remember, "just because you can doesn't mean you should". Just because you can edit Joomla! ACLs and view levels it doesn't means you should (unless you are perfectly aware of what you're doing and have a tested, just-taken backup just in case).
As you clearly understand now, this whole issue has NOTHING to do with Akeeba Backup. It's either a Joomla! bug (which I helped fix!) or a user error on your part. Of course I am not dumb! I would never knowingly publish software which could completely destroy a site without a warning. The one and only time I did that by accident (Akeeba Backup 3.1.3) I immediately unpublished the offending release and published a fixed version, complete with an apology about my mistake (it also took down my own site, if that makes you feel better). That is to say, I always try my software on my own sites. If the software is crap, my sites are the first to break. This gives me very strong incentive not to drop the ball and fix every issue I might ever cause in near-zero time. Assuming that I knowingly publish software which routinely takes down sites is not just hurting my feelings, it is enraging me - especially considering the lengths I have to go to just to make sure that this can't happen. In any case, the only potentially dangerous feature of Akeeba Backup is its restoration as, by definition, it has to overwrite files and database tables. That's why you receive abundant warnings throughout the whole restoration process so that you don't accidentally destroy your site.
So, please, before assuming that I take joy in destroying your sites and asking for money to fix them please read this page carefully, follow all of its instructions and its should be crystal clear that I do exactly the opposite of what you thing. Thanks and please stop emailing me, posting on various forums and tweeting about this alleged problem which eventually has nothing to do with my software. In fact, restoring a backup made with my software is the quickest workaround to restoring your site back to how it was before it broke.