I think i found a bug ImageGallery module, which by the way I really like (not the bug of course :).
The thing is that albums named with numbers only (eg. 2011) do not work. Link /gallery/album/2011 doesn't work (Page not found). When I renamed the album to archive 2011(/gallery/album/archive-2011) everything worked as it should.
I am using silvertstripe 2.4.7, ImageGallery 1.0.
I hope this helps.