psych gem にはまった(redmine 2.4.1 が動かなかった)

$ sudo /usr/local/ruby200/bin/ruby script/rails server webrick -e production
/usr/local/ruby200/lib/ruby/2.0.0/i686-linux/psych.so: warning: already initialized constant Psych::Parser::ANY
/usr/local/ruby200/lib/ruby/2.0.0/i686-linux/psych.so: warning: already initialized constant Psych::Parser::UTF8
/usr/local/ruby200/lib/ruby/2.0.0/i686-linux/psych.so: warning: already initialized constant Psych::Parser::UTF16LE
/usr/local/ruby200/lib/ruby/2.0.0/i686-linux/psych.so: warning: already initialized constant Psych::Parser::UTF16BE
/usr/local/ruby200/lib/ruby/2.0.0/psych/syntax_error.rb:5:in `<module:Psych>': superclass mismatch for class SyntaxError (TypeError)
        from /usr/local/ruby200/lib/ruby/2.0.0/psych/syntax_error.rb:1:in `<top (required)>'
        from /usr/local/ruby200/lib/ruby/2.0.0/psych.rb:1:in `require'
        from /usr/local/ruby200/lib/ruby/2.0.0/psych.rb:1:in `<top (required)>'

というエラーが出て redmine 2.4.1 が動かなかった。
ググったら https://github.com/burke/zeus/issues/409 で似たようなお困りを
見つけたので、試しに psych 2.0.1 の gem をアンインストールしたら redmine が動いた orz。