The current blosxom version does no longer support using names for months, such as /2008/Dec/12. This (probably hardly used) feature was broken in rev 1.67 [1], 8 months ago.
The attached trivial patch fixes this.
[1]: http://blosxom.cvs.sourceforge.net/viewvc/blosxom/blosxom2/blosxom.cgi?r1=1.66&r2=1.67
Looks fine on a first glance. Will probably intergrate it into the next minor release (which probably will be 2.1.3)