Are you connecting with the correct Add-In version to the corresponding QC version? So connecting with QC 10 Add-In to QC 10 environment or QC 11 Add-In to QC 11. Sometimes you cannot mix these because the client components cannot understand what the different versioned server is sending.
It looks like either your server is 11 version or you have 11 version Add-In installed. Or then you have client components from 11 or higher registered to you computer. Could this be the case? If so connect to the QC 10 environment using Internet Explorer or QC Explorer to register the components. (in QC 11 or higher the registration is done separately from the QC Tools menu)
It looks like either your server is 11 version or you have 11 version Add-In installed. Or then you have client components from 11 or higher registered to you computer. Could this be the case? If so connect to the QC 10 environment using Internet Explorer or QC Explorer to register the components. (in QC 11 or higher the registration is done separately from the QC Tools menu)