Problem with background in columns when activating All in One Calendar plugin

Hi, I have a problem with the All in One Calendar plugin. It’s not something critical but the problem is that when I activate the right column widget, the center column white background practically dissappears minimizing itself to a thin white stripe across the top of the column that is hardly visible.
I have no idea why this is happening and I can’t get it to work.
If I disable the widget, everything goes back to normal.
Any ideas?

Here’s some of the css code for the theme that I’m using. I’m not good at css so I would appreciate some help.
Thanks
#columns { width:928px; background:#fff; border:1px solid #ebebeb; margin:0 auto; padding: 0 20px; }
/*=== Center Column ===*/
#centercol { float:left; width:633px; padding:10px 0; }
#centercol img { margin:0 auto; padding:0; }
/*=== Right Column ===*/
#rightcol { float:right; width:275px; padding:10px 0 0 0; }
.small_link { color:#656464; text-decoration:none; font:normal 14px Arial, Helvetica, sans-serif; padding:5px 0; margin:0 auto; }
#centercol .box { border:1px solid #dbdbdb; padding:18px;}
/*=== Sub Columns ===*/
.subcols { height:1%; }
.col1, .col2 { float:left; width:285px; }
.col2 { float:right; }

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