"Class DrupalFile5Migration no longer exists"

I'm defining a file migration exactly as is suggested here: https://www.drupal.org/node/1819704 but I get this error all the time "Class DrupalFile5Migration no longer exists", I already tried deregistering the migration and registering again and also creating another one with a different name.

Edit: if I change the class name to "DrupalFile6Migration" it gets registered just fine but my source is Drupal 5 not 6.

Edit2: Just tried registering a migration from the UI, but it shows the same error message and the file migration is not displayed:
"Class DrupalFile5Migration no longer exists"
"Migration dd2bbc539File is not a valid Migration dependency."