Just for everybodies information: I had some problems with corrupted notebooks, which I could not really solve. Now I found a Wolfram tool and procedure to repair such a notebook. It does the job:

See: How do I repair corrupted notebooks using the AuthorTools package?

Like any files, Wolfram Language notebooks can be corrupted, making them unreadable. In general, we recommend backing up important work on different media. In case notebooks become corrupted, Mathematica provides a tool to help restore them using the AuthorTools package. Restoring a notebook removes any corrupted sections of the notebook, allowing it to be opened and read. Follow these steps to o repair a corrupted notebook:

1) In a new notebook, load the AuthorTools package and open the NotebookRestore dialog by executing:

Quiet[<<AuthorTools`]
NotebookRestore[]

Please note that the tick mark after AuthorTools is a back tick character (`), not an apostrophe (‘). The back tick character is available underneath the tilde key on most keyboards.

2) Select the notebook you want to restore in the NotebookRestore dialog. Mathematica will open a new notebook with what could be restored from the original one. We recommend saving the new notebook with a new name.