thx for the fast answer!
i can not see what might be the caution for the problem. i changed meanwhile the mouse and the keyboard, but the problem is still there.
there is -one- more side, where i have the problem sometimes. it is the side of a german news magazine called “spiegel online”. all other sides i have visited do not have the problem.
it only happens with chrome, not with FF or edge. so i deactivated all chrome extensions, but that is also not the problem.
i also searched for a virus, cleaned the cache, but the problem is still there…
so i decided to ask here.