Corrupt base and writer files

Hello,
I have a writer and a base file, I get an error message for both when I try to open them.
I already tried everything possible, including opening it with Microsoft word and trying a repair software.

Does anybody here know what else I could do?

Many thanks.

As only you knows this message I / we can only give general directions.
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.odt files as well as .odb are essentially zip-archives, so you may try to open them with a program for archives.
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It is always a good idea to make copies BEFORE you start “repairing” a file, so your next attempt can get a clean start when the first try fails.
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Some internals to start:

You could consider to tell us the error message. Some are not very helpful " general i/o error" but some give a clear hint.
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Another option would be to upload the files for inspection/make them accessible, if the contents are no secrets.
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But if you are right and you really

then you should search your backups or recreate the documents.

Here is the error (in German):

Die Datei “xxx” ist defekt und kann deshalb nicht geöffnet werden. Libre Office kann versuchen, die Datei zu reparieren.
Der Defekt kann durch eine Manipulation am Dokument oder durch einen Fehler bei der Datenübertragung verursacht worden sein.
Sie sollten dem Inhalt der Datei nicht vertrauen.
Das Ausführen von Makros wird in diesem Dokument deaktiviert.

Soll Libre Office die Datei reparieren?

Translation:

The file “xxx” is defective and therefore cannot be opened. Libre Office can try to repair the file. The defect may have been caused by tampering with the document or by an error in data transmission. You should not trust the contents of the file. Macro execution is disabled in this document. Should Libre Office repair the file?

Repair doesn’t work.

The approach with ZIP doesn’t work. I used backups, but the system copied the corrupted file to the backup.

Where did you save the file? In home folder of your own system?

Are there any objections to share the file(s)?