KDE taskbar grouping not showing list

After a recent KDE upgrade, the taskbar grouping did not work anymore. Instead of e.g. a list of open files in gvim i had to click multiple times on the entry in the taskbar to cycle through the list of windows.

I couldn’t find the solution on google (most likely because I use the wrong terms to describe this feature) so here is a blog post to help people like me.

The solution: Right click -> Task Manager Settings -> Behaviour -> Clicking grouped Task -> Shows textual list.