It's a standard problem with my STL. As a standard procedure, I cool to zero for at least 30mins, then to my set point after that. The moisture/ frosting issue never then results. Peter Ward fitted additional heating to the window, but this only helps after 15-30mins of being on. It's been excellent ever since.
This may be a common issue amongst other brands and models. It must be quiet a severe environment in there! Not much moisture has to get in to cause grief.
Great you worked it out Rolf.
Steve