Difficult one to answer, Dirk.
Assuming you fly FSX (on FS9, IvAp does not drop down from the add-ons menu, but installs next to it), I can only say that I would do exactly the same thing: uninstall and install again.
Maybe you need to first uninstall from the configuration manager, and try to locate any remnants of IvAp after that in the "Program files > IVAO > IvAp v2" folder. If there are any, delete them manually.
Next, before re-installing, make sure you have the latest, most recent copy of IvAp (2.0.2 of december 25th, 2012, at this moment), and install that one. Carefully read all screens!
As this is not EHM software, we are mere end users like yourself, and can only advise from experience. And my axperiences with IvAp so far have been pretty much OK. So I have no real "lessons" to share here today. Sorry for that.
I do hope you get IvAp up and running soon ...