sidebar missing when editing panel page (plainscape theme)

Hello.

I'm having a problem with panels using plainscape. When open the edit section of a panel page, the side bar that appears on the left side (that with the options "Summary", "Variants", "Panel", etc) is missing. With Garland and iTheme is OK.

I've attached two images showing the error in plainscape, and how it looks in garland.

Yet, i've posted an issue (http://drupal.org/node/690406#comment-2500408) at Panels page, and they told me that the problem could be with the theme.