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Mandatory information about my setup:

Have I read the related troubleshooter articles above before posting (which pages?)? No
Have I searched the forum before posting? No
Have I read the documentation before posting (which pages?)? No
Joomla! version: (unknown)
PHP version: (unknown)
MySQL version: (unknown)
Host: (optional, but it helps us help you)
Akeeba Backup version: (unknown)

EXTREMELY IMPORTANT: Please attach a ZIP file containing your Akeeba Backup log file in order for us to help you with any backup or restoration issue. If the file is over 2Mb, please upload it on your server and post a link to it.

Description of my issue:
I installed Akeeba 3.3.11 to one of my client's website (Joomla 1.5.15). The installation generated an error below and now I cannot access to the backend of the admin panel.

JInstaller::install: File '/var/www/vhosts/timeline2012.net/httpdocs/tmp/install_4f107a6f1aea3/plg_oneclickaction/language/da-DK/de-DE.plg_system_oneclickaction.sys.ini' does not exist.

I followed the tips on the troubleshooting article on your website and removed the following files but I still cannot access to the backend.

administrator/modules/mod_akadmin
plugins/system/srp.php
plugins/system/srp.xml
plugins/system/oneclickaction.php
plugins/system/oneclickaction.xml
plugins/system/aklazy.php (did not see this)
plugins/system/aklazy (did not see this)
plugins/system/akeebaupdatecheck.php
plugins/system/akeebaupdatecheck.xml

Sorry, I forgot to answer the following questions:

Have I read the related troubleshooter articles above before posting (which pages?)? Yes
Have I searched the forum before posting? Yes
Have I read the documentation before posting (which pages?)? Yes
Joomla! version: 1.5.15
PHP version: 5.2.12
MySQL version: 5.0.89
Host: psa v8.6.0_build86080930.08 os_CentOS 5
Akeeba Backup version: 3.3.11

I didn't install anything else. Akeeba backup was the first extension I tried to install...

Oh, I remember now. I tried to change the output folder for Akeeba backups. I created a new folder in the site root and then went to Akeeba Admin panel from Components. But I was unable to change the output folder. That's when I lost my access to the backend.

Now when I try to access the backend, this is what I see. The snapshot image is attached.

Thank you for your help!

UPDATE:
As it turned out the problem had already been solved by the suggested solution by Nicholas (deleting the akeeba related files). This is a new client and I didn't know it until now but they are using for their website a Content Delivery Network service called ProCDN, which caches the website. All I was seeing was a cached page! This is why I was still seeing the same error page even after removing all the files from the backend. When my client tried, he was able to login to the backend of Joomla. He told me that the cached images would go away after a week!