Access to (private) file system suddenly stopped working (Access denied)

Help! From one minute to the next, access to the private file system stopped working. For any file (images, PDFs, etc.) Drupal simply returns

    Access denied

    You are not authorized to access this page. 

This is for both anonymous and authenticated users (including user #1). Access to everything else works fine.

What I've tried so far:

• Clear all caches
• Rebuild permissions (under admin/content/node-settings)
• Run cron
• Restart browser etc.
• Try different browser, computer, etc.

I can still *upload* files (they appear in the directory set as the private file system), but then accessing them won't work. There's nothing in the Apache error logs and in the watchdog logs there's nothing unusual, except from one minute to the next there's a huge number of "access denied" entries.

Any ideas on how to track this down would be more than welcome...

r.

Some details on my installation:

Drupal	6.17
Access to update.php	Protected
Configuration file	Protected
Cron maintenance tasks	Last run 23 min 15 sec ago
You can run cron manually.
Database updates	Up to date
Drupal core update status	Up to date
File system	Writable (private download method)
GD library	bundled (2.0.34 compatible)
Image module directories	Exists (/home/user/drupal-files/images).
Image toolkit	The gd toolkit is installed.
Module and theme update status	Up to date
MySQL database	5.0.90
MySQL database for event module	5.0.90
PHP	5.2.13
PHP memory limit	100M
PHP register globals	Disabled
Unicode library	PHP Mbstring Extension
Update notifications	Enabled
Web server	Apache
jQuery Update	1.2.6