Thanks for the suggestion, but it didn't work.
What I now did is:
upload aall new WP 2.6 files
replaced pluggable.php (2.6) with pluggable.php (2.5.1)
going to upgrade.php in the browser again
It did work! I can now enter the Admin area again and could reactivate all the plugins which where deactivated before upgrading.
After these things where successful, I tried to take the pluggable.php from the 2.6 installation package and copied it into the includes folder to replace the 2.5.1 version.
What happend? Same mistake again. My Browser showed:
Parse error: syntax error, unexpected T_STRING in ... /wp-includes/pluggable.php on line 1265
So now I have a 2.6 installation with a pluggable.php that misses a lot (7KB) of new code.
Line 1265 in pluggable.php is around the paragraph:
// If the stored hash is longer than an MD5, presume the
// new style phpass portable hash.
if ( empty($wp_hasher) ) {
require_once( ABSPATH . 'wp-includes/class-phpass.php');
// By default, use the portable hash from phpass
$wp_hasher = new PasswordHash(8, TRUE);
}
This has to do something with the new passwords and the responding information in wp-config.php, right?
I don't know what to do now.