Update hangs "update will be prepared"

I wanted to install the update Choosing “download” does not work. It hangs (more than 10 minutes) and I only get the message “update will be prepared” (Aktualisierung wird vorbereitet). This is my second attempt. The first did end in the message “try again later”. So the update should already be downloaded.
What’s going wrong here.
Xperia XA2