As reported on wiki, the usercontribs module of the Api no longer accepts usernames that are prefixed with User:, which is a breaking change, that caused the MediaWiki-Bot to break.
The change that caused this was traced by @Anomie to Change d3da5e08d3
This was previously undocumented behavior of the usercontribs module. We probably should properly deprecate this after a short term workaround ?