Plesk updates don't install automatically, "error occurred while processing your key"

[Knarf]

Recently I installed Plesk 11.0.9 Update #19.
It has a valid license key of my provider Strato, with a Key expiration date of Nov 24, 2012.

Every day however, I get e-mails with the text:

Unable to update . An error occurred while processing your key. You can try updating it later.

I tried this:
http://kb.parallels.com/en/111198
I don't know if this KB article applies to Plesk 11 as well, but I tried it and get this message:

/usr/local/psa/bin/sw-engine-pleskrun -f ./test.php
ERROR: Zend_Exception
No entry is registered for key 'log'

0: Registry.php:147
Zend_Registry::get(string 'log')
1: common_func.php3:5267
notifyAboutKeyUpdateResults(string '', boolean false, string 'No entry is registered for key 'log'')
2: test.php:20

What can I do to solve this problem?