Downloading files to a network drive fails

If I want to download the attachment to a mail, I can only do so to a directory on my local hard drive. If I download the attachment to a network drive, the download only seems to work. The file is created, but has no content (0 bytes). If I open the attachment directly from the mail with a viewer, I can save the file on a network drive without any problems.
Here are some Bilder dazu.

Download to a local folder:

Download to a network folder:

As you can see in the last screenshot, the files on the network drive have the size 0 bytes. This is regardless of whether I download the files individually or collectively.