For me, absolutely nothing happens if I pres alt-C (Object Mode, Text selected). what is wrong here (blender 2.9)

In Blender 2.8 and higher, Alt+C isn't assigned to a command. But you can assign to it to the menu command Object > Convert To > Mesh. (Right-click the menu entry to the assign shortcut.)

Convert To Mesh works in Object Mode when the text is selected.