Blog not showing sidebar and only one post in IE 8 & 9.

I think that a third party plugin causes this issue because something adds a lot of IE related code to the website. I.e. code like

<!--[if gte mso 9]><xml><br />
<w:worddocument><br />
<w:view>Normal</w:view><br />
<w:zoom>0</w:zoom><br />
<w:trackmoves></w:trackmoves><br />
<w:trackformatting></w:trackformatting><br />
<w:punctuationkerning></w:punctuationkerning><br />
<w:validateagainstschemas></w:validateagainstschemas><br />
<w:saveifxmlinvalid>false</w:saveifxmlinvalid><br />
<w:ignoremixedcontent>false</w:ignoremixedcontent><br />
<w:alwaysshowplaceholdertext>false</w:alwaysshowplaceholdertext><br />
<w:donotpromoteqf></w:donotpromoteqf><br />
<w:lidthemeother>EN-US</w:lidthemeother><br />
<w:lidthemeasian>X-NONE</w:lidthemeasian><br />
<w:lidthemecomplexscript>X-NONE</w:lidthemecomplexscript><br />
<w:compatibility><br />
<w:breakwrappedtables></w:breakwrappedtables><br />
......

which uses the conditional tag

<!--[if gte mso 9]>

to “hide” the information from other browsers. But obviously IE8 and IE9 are affected and the code seems to break them.