The model BG 1805/E has a viewing window with double glazing. Depending on the ambient temperature and type of food, condensation may collect in the space between the panes during roasting/baking. With larger quantities, it may take a little longer for the water to evaporate again. Unfortunately, it is generally not possible to completely prevent the glass from misting up.
However, the inner door glass can be opened for cleaning. With a few simple steps, you can loosen the pane and wipe off the condensation. Instructions on how to do this can be found in the oven operating instructions.