I have the following entry in crontab:
0 5 * * * /bin/bash -l -c \'export RAILS_ENV=my_env; cd /my_folder; ./script/my_script.rb 2>&1 > ./log/my_lo
Your redirection order is incorrect. Stderr is not being redirected to the file, but is being sent to stdout. That's what you must be receiving in your mail.
Fix the redirection by changing your cron job to:
0 5 * * * /bin/bash -l -c
'export RAILS_ENV=my_env;
cd /my_folder;
./script/my_script.rb > ./log/my_log.log 2>&1'