Jamiroquai.com and the official website of the Konig Pilsener Arena in Germany have confirmed that tonights (Tuesday's) concert has been cancelled.  Jamiroquai.com writes:

Unfortunately owing to illness tonights Jamiroquai concert in Oberhausen has had to be cancelled. We apologise for any inconvenience caused and will advise of the rescheduled date for the concert as soon as we can.

The Konig Arena website says (English translation 'attempt' courtesy of Google):

Because of an acute flu-like infection of the singer Jay Kay of Jamiroquai has today's concert will be canceled. The organizers are seeking a new date and hope to announce these shortly. Tickets remain valid or can be returned to the box offices, where they were acquired.

The band are set to perform in Rotterdam tomorrow (Wednesday) and in London on Friday.  These (and all future dates) are currently not affected.  For the latest information please check jamiroquai.com.