Hi Loed,
for me the best way to test the connection is to activate an "easy" ( at example a Text) DataSource from Business Context and test at first with this DataSource. If it is OK, you have a large chance that the connection is OK.
If not, please check the TA WE20 on both side of connection. Under "Partner Type LS" check for your Partner System and in the tab "Classification" the partner status must have status "A" aktive.
Peter