It worked after resetting the trac server. Both the wiki preview time-out and the ticket preview time-out are now at 8s.
As a test, I reset the
to 2s and didn't restart the server (I'm using tracd).
The wiki preview time-out picked up the change immediately, but the ticket preview time-out didn't; it stayed at 8s.
It seems that the auto-reload of the configuration is not applied to the
ticket preview time-out.
But then, I discovered what I did: I kept trying the same ticket without refreshing the browser. As soon as I refreshed the browser or tried a different ticket, the ticket preview time-out was updated to the current value. The same happens when editing a wiki page. I had to cancel the edit or re-load the page before the new time-out was picked up.
So it was a user error after all ;-)