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