mp3 files cannot be uploaded

When I try to upload mp3 files to the media center, there is the following error notification (in German); the files cannot be uploaded:

Unerwartete Antwort des Servers. Die Datei wurde möglicherweise erfolgreich hochgeladen. Schau in der Mediathek nach oder lade die Seite neu.

I would be glad to receive a solution for this problem. If it is useful, I can provide the text of the error log.

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