I'm unable to query AGFA Impax 6.x with the Query tool

Make sure Agfa Impax authorizes Orthanc to perform Query Retrieve.

Then, run Orthanc in verbose mode and check the logs. It will tell you if the C-Find association can be completed.

If you still do not understand your problem at that point, stop Orthanc and run findscu (pretending it is Orthanc) with the -v option and you shall get plenty of information about the DICOM association and hopefully the reason why it fails. Eventually, post the output of these logs here so we can help you.