gistfile1.txt

mbc$ knife cookbook upload -a
INFO: ** activemq **
INFO: Saving activemq
INFO: Validating ruby files
INFO: Validating templates
INFO: Syntax OK
INFO: Generating Metadata
WARN: HTTP Request Returned 404 Not Found: No routes match the request: //sandboxes
/Users/mbc/.rvm/rubies/ruby-1.8.7-p174/lib/ruby/1.8/net/http.rb:2097:in `error!': 404 "Not Found" (Net::HTTPServerException)
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/lib/chef/rest.rb:217:in `api_request'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/lib/chef/rest.rb:268:in `retriable_rest_request'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/lib/chef/rest.rb:198:in `api_request'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/lib/chef/rest.rb:112:in `post_rest'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/lib/chef/knife/cookbook_upload.rb:105:in `upload_cookbook'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/lib/chef/knife/cookbook_upload.rb:197:in `catch_auth_exceptions'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/lib/chef/knife/cookbook_upload.rb:105:in `upload_cookbook'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/lib/chef/knife/cookbook_upload.rb:63:in `run'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/lib/chef/cookbook_loader.rb:160:in `each'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/lib/chef/cookbook_loader.rb:159:in `each'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/lib/chef/knife/cookbook_upload.rb:61:in `run'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/lib/chef/application/knife.rb:115:in `run'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/gems/chef-0.9.6/bin/knife:25
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/bin/knife:19:in `load'
	from /Users/mbc/.rvm/gems/ruby-1.8.7-p174/bin/knife:19

I'm getting this issue too (Mac OS X 10.6).
Following this article (http://help.opscode.com/faqs/start/user-environment), except pointing my knife config to my own Chef Server.
Any luck there?

I have a feeling it might be chef 0.9.6 pointing to chef 0.8 issue?

update: It's a versions issue - see: http://wiki.opscode.com/display/chef/Upgrading+Chef+0.8.x+to+0.9.x

solution is either to upgrade the chef on your server, or downgrade the chef on your repo.
I downgraded the chef on my repo (local workstation) to version 0.8.16 & everything seems to work