What does the error message "There was an error updating the list" mean? I got it while trying to add someone to a new, private, list using twitter's web site with Safari.
This is one of those generic error messages that are used to cover all sorts of issues.
It allows the website or web app to provide a level of abstraction so that users only need to know that something is up and to either wait a bit before they retry or follow-up with support or the developers.
It's deliberately vague because the user doesn't need to know the exact details of what's gone wrong which may include:
These things are usually temporary and retrying later usually will go through fine.
If not, then the error message will show more details or there will be a related post on the site or its blog to detail what's happening and/or why.