A few weeks ago Twitter stopped supporting this URL convention to grab XML feeds of a user's timeline:

http://twitter.com/statuses/user_timeline/{{SCREEN_NAME}}.(xml|json)

Fortunately, it's still possible to fix your apps with a drop-in replacement, which was probably always considered the "right way" to grab them anyway:

https://api.twitter.com/1/statuses/user_timeline.(xml|json)?screen_name={{SCREEN_NAME}}

The bad news is it doesn't look like this will work for long. REST API 1 is now deprecated and the new REST API 1.1 is set to replace it any day now:

Version 1 of the REST API is now deprecated and will cease functioning in the coming months. Migrate to version 1.1 today.

Unfortunately, this migration path isn't as easy as just dropping in a new URL. It appears even the read-only timeline XML/JSON feed requires an OAuth authentication token.