The complete .sidebar was positioned below the .content element. I guess because there are some other elements within the sidebar that require a minimum width. I changed style1.css including the search form width:
/* Breite ändern für größere Bilder und schmalere Sidebar */
/* 640px wide + 45px right padding = 685px */
/*.content { padding: 0 45px 0 0; width: 685px; } */
.content { padding: 0 10px 0 0; width: 620px; }
/* Was 280px width, must remove 50px to keep content inline*/
/*.sidebar { padding: 20px 0 40px 25px; width: 230px; } */
.sidebar { padding: 10px 0 10px 15px; width: 260px; }
.content img { max-width: 100%; }
/* Suchformular */
.sidebar #s{
width:160px;
I may optimize the layout in the next days but it works fine for me now, thanks!`