this seems to be a bug in 64 bit TortoiseGit, see https://tortoisegit.org/issue/1675 where is is described for deleting Folders Keep local.
The original idea of how it should work seems to be what I wrote in my comment: the file is kept localy in the Directory as untracked, but should be removed from the local index. The bug seems to be fixed (see This issue was closed by revision 3a6e98b0c2e0 in the abovementioned link). Haven't tried yet.
If I don't come back here, upgrading has solved the Problem. Maybe this helps others having a similiar Problem.
thanks for all answers.
Klaus