skroddu and Badmortigan,
1) Clear the Download Cache. From the Game Launcher, go to Configure. Then when the Configuration Utility launches go to Repair and click on "Clear Download Cache".
2) Delete Addins.xml and Manifest.xml file in My Documents\\Bioware\\Dragon Age\\Settings
3) If your My Documents directory location is some other drive then C:, move it back to the default location while installing DLC. Right click on USERNAME\\My Documents and select Properties. Go to the Location tab to check or change the location, and "Restore Default" to move it back to the default location.
4) Change your Windows Region and Language settings to English(United States) temporarily.
If all else fails, you can try manually installing the DLC. Links and instructions are in section 4 of the
Redeemed DLC Troubleshooting FAQ. Even if it doesn't work, manually installing can provide useful errors to help figure out what's going on.