Since a few days I have an automatic window which pops up on the desktop startup. The window has the header

"internetseitenzugang bestätigen"

but I can not determine from which process it is created. The processes in "autostart" are not responsible, as far as I can see.
How can I identify the underlying process? Is there any tool (e.g. from KDE framework) to do this?
(I could post a screenshot here, but this forum does not allow to insert images)