No need to download anything. This can happen when something prevents update to finish in very critical part when installing/writing the new JDownloader.jar file.
Just rename JDownloader.jar.backup.1 back to JDownloader.jar and delete the update and tmp folder and the file Core.jar and start JDownloader. It will then update itself to latest available version.
Glad you could fix this by yourself!
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JD-Dev & Server-Admin
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