Have you tried right-clicking each cell and selecting the "Custom Format Cell" option.
You need the grid enabled to see this menu, or the user setting "Always Show Design Menu Items" must be enabled.
You may need to check in two places for each line as the bold line between Land and Wochentag could be a right border on the Land cell or a left border on the Wochentag cell
thanx for your answers.
Unfortunately none of them is the right one.
I parallel contacted Qlik_Support and they gave me the folllowing answer:
You can not design cell borders of dimension in a pivot chart. This is not a bug,
it works as designed !
So there's no possibility to drop the bold dimension seperator lines.