Currently the allmusic.com album links of pattern "http://www.allmusic.com/album/the-name-of-the-album-mw0123456789" are automatically shorten to "http://www.allmusic.com/album/mw0123456789" when linking to release group.
But there is a new type of link for releases (since the re-design of May 2012 I guess) which pattern is "http://www.allmusic.com/album/release/the-name-of-the-album-mr0123456789" which correspond to the release level of MB and not only to the release-group level as they used to.
So when Adding URL Relationship to a release, the link "http://www.allmusic.com/album/release/the-name-of-the-album-mr0123456789" should be shorten to "http://www.allmusic.com/album/release/mr0123456789" and the type "has an Allmusic page at" should be automatically selected as for release group.
The error message in release group should be updated, currently it says "This URL is not allowed for the selected link type, or is incorrectly formatted." when trying to add a Allmusic release link to a release group. The error message should be more explicit and explain it should be linked at release level.
And "Releases: http://www.allmusic.com/album/release/mr<ID>" should be added to http://musicbrainz.org/doc/Style/Relationships/URLs#Standardised_URLs