Linux - infinite error loop, failing to handle SRTM .zip files

We’re running an Ubuntu Linux server, 64 bit. The S1 tool box installed correctly and appears to operate as intended except for when it attempts to download srtm data as part of one of the processing ‘graphs’. It fails and attempts to download the data over and over again.

The error we get in the console window is:

I/O error reading image metadata!
ftp retrieving path/

The ftp is working correctly, the file downloads ok but is immediately re-downloaded when it reaches about 17Mb.

All input and output directories have full permissions (777).

Is this a bug or are we missing something?