My guess is that this update, from today, which introduces the version 18 of postgres, will need two rebuilds. From the web interface it’s only possible to do one rebuild, and if I’m right about that, a web interface update will always appear to fail. In fact, it didn’t fail, but only went half way.
This guess might be wrong - I can’t easily test it.
More than one person, including me, has said they no longer use the web update mechanism, because if it does fail then you need to visit the command line anyway. It’s true that it works 95% of the time, maybe more. But as it sometimes fails, I personally don’t use it.