According to La Biennale di Venezia:

"Due to recent health issues Keith Jarrett has cancelled all of his engagements for 2018.

29 September, Biennale Musica: there was enormous anticipation for the solo piano performance by Keith Jarrett, culminating in the awards ceremony for the Golden Lion for Lifetime Achievement, but recent health issues required Keith Jarrett to cancel all of his engagements for 2018, starting with the concert at Carnegie Hall in New York scheduled on March 21st and including his announced solo performance in Venice on September 29th. This timeframe affecting the Maestro’s schedule for the year has imposed a change to our programme."

La Repubblica (article in Italian) is also reporting on the cancellation.

Thanks to Jan and Vasco for the links.

I don't have any additional information at the moment. Let's hope that Keith will get better soon.