Winforms Richtextbox not properly rendered in WPF project

I have a very strange issue in my WPF project. The main window contains several wpf controls and winforms RichTextbox(don't ask me why) within 开发者_JS百科WindowsFormsHost element. Richtextbox contains text. In some cases Richtextbox is not properly rendered when loading window (the right part is white like somebody uses erase tool and clears a rectangle). This situation is not so common (~20 users / 30 000) and it probably depends on hw. It occurs on XP machines. I have tried to force sw rendering, but it didn't help. Application is built in .net 3.5 SP1. Any idea?

The problem was caused by user's unusuall dpi settings in Window. Strange, but with normal values, it works