When I set the Panel with the Fieldsets as the active item via panel_with_card_layout.getLayout().setActiveItem('form_with_fieldsets_panel') everything was shown/rendered besides of the form fields inside the Fieldsets.
After a little debugging and breakpoints fun in Firebug, I decided to search in the Ext JS Forum and luckily found the solution for that problem fast, thanks to the Forum's Google Search.
The solution is that you have to call panel_with_card_layout.doLayout() after setting the active item.
In conclusion, the following two lines should always go together when using FieldSets in a CardLayout:
Here's the note from an Ext Support Team member regarding this problem, which notes that there is another solution when you are using the Ext.TabPanel (which has the CardLayout as default Layout):
This is exactly why layoutOnTabChange was added to tabpanel. Unfortunately this fix wasn't added to card layout, so you'll have to call cardPanel.doLayout(); yourself after cardPanel.render('showcard');
Quote from http://extjs.com/forum/showthread.php?p=156627
Luckily, this is one of the rare bugs you'll run in when working with Ext JS. Pretty much everything else works as expected. :-) After all, a Friday the 13th could be worse. ;-)