The answer is possibly in /var/log/secure, although saying that I've generally found FTP logs to be lacking. The directory /var/www/vhosts/domain.de/httpdocs directory only be executable for psaserv and the username for that vhost (as seen by Plesk).
If you have ACL available (not sure for Debian 5), you could try:
setfacl -m user:USER:r-x /var/www/vhosts/domain.de/httpdocs
setfacl -m -R user:USER:rwx /var/www/vhosts/domain.de/httpdocs/test
setfacl -m -R -d user:USER:rwx /var/www/vhosts/domain.de/httpdocs/test
If you get an error complaining about the syntax, you probably need to remount the partition with ACL. Try:
mount -o remount,acl /var or
mount -o remount,acl / or whatever the relevant partition is.
If not, I'd advise not giving full permissions to the any user, and instead create a new dummy group for it to replace psaserv for that directory, so, roughly:
for i in $(lid -g psaserv | cut -f2 -d" " | cut -f1 -d"("); do usermod -aG your_dummy_group $i; done
chgrp your_dummy_group /var/www/vhosts/domain.de/httpdocs
That will let you into the directory. As for reading/writing, you'll need to do the same as above except recursively for the directory /var/www/vhosts/domain.de/httpdocs/test and with the group psacln (which, I believe but don't have an installation to test, gives full access to the directory - if not, you'll have more permissions to modify).
Obviously you could 777 the whole thing, but it would be a bad idea.