We have the following warning:

DEPRECATION WARNING: The InstanceMethods module inside ActiveSupport::Concern will be no longer included automatically.
Please define instance methods directly in CollectiveIdea::Acts::NestedSet::Model instead.
(called from include at /var/lib/redmine/lib/plugins/awesome_nested_set/lib/awesome_nested_set/awesome_nested_set.rb:58)

It doesn't do any harm on our side, but it seems related to issue #11072.

Anyway, to fix it, I followed what I found in the following answer on Stackoverflow and it did work:
http://stackoverflow.com/a/8684885
(eg. removing the "@module InstanceMethods@" declaration and the corresponding "@end@").

Usual informations:

• Redmine 2.0.3 from SVN r9959 (@branches/2.0-stable@)
• Ruby 1.8.7
• Rails 3.2.6
• Debian Squeeze