I don't know if there is a conflict and I don't think there should be one.
Panda ActiveScan was the first online scanner that I used. When I tried to use Housecall, I have never been able to download the signatures. I can download the Active X or Java controls, but not the signatures. Like your problem with ActiveScan, I get an error saying that there could be an internet connection problem or what not.
Anyways, this is just a guess. If someone posts and says that they use Housecall and Activescan, it will prove my guess wrong.