Just a few more comments to my question: If I do not start xwinclip, I can copy and paste e.g. formulas in OpenOffice, or tables from OpenOffice to an Evolution HTML email.
This is what is going on AFAICT: xwinclip detects when my Linux box is updating its clipboard and then updates the windows clipboard. At this point xwinclip updates the linux clipboard, since the windows clipboard changed. However, the linux<->windows clipboard format translations are inheritly lossy, and hence I want xwinclip to avoid them if it can. Ãyvind