I investigated a bit deeper. There are encrypted files on the disk (I examined the first characters of the file), but the check in source file DecryptAll.php, about line 226 if ($file->isEncrypted() === false) { is evaluated to true, so the file is not going to be decrypted. In the database table oc_filecache the attribute encrypted is 0. I did a sudo -u www-data /usr/bin/php /var/www/html/owncloud/occ files:scan --all but the database entry was not updated.
What am I doing wrong? Any hint?
Thanks,
Markus