I have just updated an existing SQL Server Express server to SQL Server
Developer Edition 9.0.1399. In general this has worked fine, but I cannot
create a Maintenance Plan, either using the Wizard or by using "New
Maintenance Plan..." The latter brings up a dialog saying that:

Exception has been thrown by the target of an invocation. (mscorlib)
Additional Information: An OLE DB error 0x80004005 (Client unable to
establish connection) occurred while enumerating packages. A SQL statement
was issued and failed.

while the SQL Server log says:

Error: 17828, Severity: 20, State: 3.
The prelogin packet used to open the connection is structurally invalid; the
connection has been closed. Please contact the vendor of the client library.
[CLIENT: <local machine>]