Is there any possibility to migrate data from Redmine to Mantis BT ?
Is this a loss-less operation ? I've read, that Mantis contains less project management tools than Redmine. Thus mantis could be a subclass of Redmine -> therefore would we loose some project data ?
Mantis is just an Issue Tracking System and doesn't have any built-in project management features. There are certain plugins which may help you with PM, but there is no way to migrate PM data to those plugins automatically.
However, you can export issues from Redmine in CSV format and then import to Mantis using the Csv_Import plugin available here: https://github.com/mantisbt-plugins/csv-import