I was testing migrate_from_trac.rake and found that it was not importing the user database from trac.
I substituted ".save" with ".save!" in a few places in the script and found this error:

Validation failed: Password is too short (minimum is 8 characters)
/usr/local/lib/ruby/gems/1.9/gems/activerecord-3.2.13/lib/active_record/validations.rb:56:in save!'
/usr/local/lib/ruby/gems/1.9/gems/activerecord-3.2.13/lib/active_record/attribute_methods/dirty.rb:33:insave!'
/usr/local/lib/ruby/gems/1.9/gems/activerecord-3.2.13/lib/active_record/transactions.rb:264:in block in save!'
/usr/local/lib/ruby/gems/1.9/gems/activerecord-3.2.13/lib/active_record/transactions.rb:313:inblock in with_transaction_returning_status'
/usr/local/lib/ruby/gems/1.9/gems/activerecord-3.2.13/lib/active_record/connection_adapters/abstract/database_statements.rb:192:in transaction'
/usr/local/lib/ruby/gems/1.9/gems/activerecord-3.2.13/lib/active_record/transactions.rb:208:intransaction'
/usr/local/lib/ruby/gems/1.9/gems/activerecord-3.2.13/lib/active_record/transactions.rb:311:in with_transaction_returning_status'
/usr/local/lib/ruby/gems/1.9/gems/activerecord-3.2.13/lib/active_record/transactions.rb:264:insave!'
/usr/local/www/redmine/lib/tasks/migrate_from_trac.rake:280:in find_or_create_user'
/usr/local/www/redmine/lib/tasks/migrate_from_trac.rake:435:inblock in migrate'
/usr/local/www/redmine/lib/tasks/migrate_from_trac.rake:428:in each'
/usr/local/www/redmine/lib/tasks/migrate_from_trac.rake:428:inmigrate'

Changing the default password for created users from 'trac'
to 'tracmigratetoredmine' fixed the problem.

See attached patch.

Related to: #14567