Got it!
However, I'm not completely sure what solved the problem now. This is what I
did:
# Setting the startup type of "MSSQL$SBSMONITORING" from disabled to manual.
# Download SQLWMSDE-KB960082-x86-DEU.exe and start it with these parameters:
SQLWMSDE-KB960082-x86-DEU.exe /upgradesp sqlrun DISABLENETWORKPROTOCOLS=1
DISABLEAGENTSTARTUP=1 DISABLETHROTTLE=1 BLANKSAPWD=1 INSTANCENAME=Sharepoint
REINSTALL=ALL REINSTALLMODE=VEMUS /l*v %windir%\WMSDE.log
See
http://support.microsoft.com/kb/967092 for details. And mind the
backslash in front of wmsde.log - it's missing in the knowledgebase article.
Setup won't work without it.
Result: Update is still offered for installation on Microsoft Update.
Installing once again via Microsoft Update is succesful.
Thanks for your support!
Lennart