Hi,
I've tested our jDownloads extension and I've found only a minor bug with the file title not being added to URL correctly, but this shouldn't cause the problem you describe. Anyway you can update the extension using Manage Extensions page in JoomSEF.
If it doesn't help, please post the type and parameters of your home link menu item from Joomla's Menu Manager.

Thank you for your .

But the extension could not be the problem, because the error is the same, when I disable it (or uninstall it)...

So there must be another reason for this problem :/

Please post the type and parameters of your home link menu item from Joomla's Menu Manager.
What URL is displayed in your browser's address bar when the 404 error is displayed? Check if this URL is created in JoomSEF's URLs Manager and if it is, please post its Real non-SEF URL.

Hi escort,
What page causes you the 404 error? Is it the homepage or some other link?
If it's a homepage, please post the type and parameters of your home link menu item from Joomla's Menu Manager.
If it's some other link, please post the SEF URL and its corresponding Real non-SEF URL from JoomSEF's URLs Manager.

My problem is also solved :)

Just a setting fail ;)