The problem seems to be, that the proxied backend server that is cached via mod_disk_cache originally delivers HTTP status 301 and the Location http://www.beach-clothing.com/where-to-buy/, but once cached mod_disk_cache delivers HTTP status 200 instead of 301 (but correctly redelivering the Location header). I have not proved this for myself so far, but this seems the problem to me.
This wouldn't surprise me one bit. The 2.1 branch has seen quite a bit of churn in this area.
Any chance you could give 2.1 a go and see if that works correctly?
Sander
