I don't know why, but somehow "ImageMagick" gained the status of being my default PDF viewing application (I didn't set it manually, maybe it came with the installation of iscan or plex).
Now, when I try to set Envince - or any other viewer - via [right-click on a PDF] > [Properties] > [Open with...] it happens nothing! ImageMagick remains as associated application.
Is there any way to get rid of that association? I guess ImageMagick is somehow made the systemwide standard on root level.