Drupal 7 Migrate Module: Migration not working with result "Processed 0 (0 created, 0 updated, 0 failed, 0 ignored)" even if there are entries to process

Some days ago I had to program a bigger import for a Drupal 7 Forum, including several hundreds of thousands of entries and comments. I decided to use the Drupal migrate.module as helper for this import and it was a good decision!

migrate.module made things much easier and reduced the amount of custom import code.