Hi Florian,
I think this was a bug in gpredict 1.3 that has been fixed 3 years ago
- unfortunately, no stable release has been made since then.
You could try building the latest gpredict from the source tree. It's
not that difficult and I'm sure I or somebody else can guide your
through it if you want to try. I'm in the middle of moving right now,
so it may take several days for me to answer during the coming weeks.
Alex