Hi Florian,
your problem is related to the Tab panel's principle: the tabbed panels are child panels of the panel onto which are located, and they have different handles, so you cannot acces control on the main panel with the handle of the tabbed one.
In your SICKNESS_FUNC callback modify the code as follows:
GetPanelAttribute (panel, ATTR_PANEL_PARENT, &parentPanel);
SetCtrlVal(parentPanel, PANEL_STRING, "HALLO");
In case you need to heavily access controls on different panels inside your program, you could consider storing their handles into global variables to access them more easily.
Roberto