insecure: can't modify hash

A weird problem occurs (only on the prod server, not locally), when i
try to register a new user with restful_authentication.

Once in a while, i get the following error when trying to sign up as a
new user:

Insecure: can't modify hash

app/models/user.rb:113:in `make_activation_code'
app/controllers/users_controller.rb:30:in `create'
/usr/bin/mongrel_rails:16:in `load'

Im really clueless on what's causing it. Even more strange is that it
doesn't right after a fresh deploy, and it doesn't happen locally. My
first reaction was to run rake db:sessions:clear in case there was some
unwanted persistence in the sessions, but it doesn't seem to come from

If any knows about that error and could help... thanks :!