Processing files which are not in backup sources

I‘m using Duplicati as a windows service under Windows 7 and have updated from 2.0.3.5 to 2.0.3.8 on 30.6.2018 and to 2.0.3.9 on 2.7.2018. The backup job is performed with --snapshot-policy=on.

Since then I have seen the phenomenon of negative numbers at the progress bar described here:

What is worse, however, is that data has been backed up that is not in backup sources. Now there are tons of useless GB in the backup set. :frowning:

Questions:

  1. How can I safely rollback to 2.0.3.5?
  2. Can the data that should not be backed up be deleted from the backup set? The file paths are known.