WebMail/WebDocuments: ZIP-Files not downloadable

When a user receives an e-mail in WebMail containing an attached zip-file, this zip-file cannot be downloaded in WebMail. The browser initiates the download (showing it at the bottom of the browser), but the download doesn't progress and times out after a while. This is true for any type of browser. I can safe the zip-File in WebDocuments (where it shows the correct size), but also from there, the download fails.

Other types of attachments (office, images etc.) are not affected.

The zip-file exists in \~gw\attachments\, but with a size of 0 bytes. However, the size is displayed correctly in WebMail and WebDocuments.

The file can be opened however when it is received by a local mail client.