message Missing or not readable file: .users/userdb.dat during migration

I am trying to migrate a big Gallery 1 (6.000-7.000 images) to Gallery 2.
Gallery 2 is installed and the Gallery 1 data is also available on the same computer.

I selected all the existing Gallery 1 users and then clicked on the Prepare Import button.
Then I get:

Security Violation

The action you attempted is not permitted.

Back to the Gallery
Error Detail -
Error (ERROR_BAD_PARAMETER) : Missing or not readable file: .users/userdb.dat

* in modules/migrate/classes/Gallery1DataParser.class at line 66 (GalleryCoreApi::error)
* in modules/migrate/classes/Gallery1DataParser.class at line 289 (Gallery1DataParser::loadFile)
* in modules/migrate/ConfirmImport.inc at line 1553 (Gallery1DataParser::getUserUids)
* in modules/core/SiteAdmin.inc at line 201 (ConfirmImportView::loadTemplate)
* in modules/core/classes/GalleryView.class at line 300 (SiteAdminView::loadTemplate)
* in main.php at line 445 (GalleryView::doLoadTemplate)
* in main.php at line 94
* in main.php at line 83

System Information
Gallery version 2.2.5
PHP version 5.2.5 apache
Webserver Apache
Database mysqlt 5.0.51a-log
Toolkits NetPBM
Operating system OpenBSD appeal 4.3 GENERIC#4 i386
Browser Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.8.1.15) Gecko/20080623 Firefox/2.0.0.15

Gallery version: gallery 2.2.5
Webserver (with version): Apache 1.3.29
Database (with version): mysql 5.0.51a
PHP version (eg 4.2.1): 5.2.5
Graphics Toolkit(s): netpbm 10.26.46
Operating system: Web browser/version: Windows XP Firefox 2.0.0.15
G1 version (for migration bugs): v1.5.4

Apparently Gallery2 can read the userdb.dat to show which users are there but then it suddenly doesn't have access anymore.
Any idea what is causing this?

Daniel