Lol, things work in mysterious ways!
It turns out that to get the Postgres ODBC driver to work properly I had to read from the table before inserting data into it. In all other applications I did read from the DB as well as write. In my current application I only write, and that was the difference.
So before I write, I have to do a little reading:
someDataSetTableAdapters.amunds_dataTableAdapter taData =
new someDataSetTableAdapters.amunds_dataTableAdapter();
someDataSet sds = new someDataSet();
someDataSet.amunds_dataDataTable adt = sds.amunds_data;
taData.Fill1(adt);
int x = taWeatherData.Insert(
(decimal)data.X,
(decimal)data.Y,
(decimal)data.Z);
Man, I've googled this one! Hope it will help somebody sometime in the future.