Do you have any custom javascript that uses jquery? The problem is not likely caused by the jquery script itself but by some other script that depends on it. If you are using other jquery plugins that are not compatible with the latest version of jquery you can set it to use an older version from Web.config.
Note that by default we load jquery from the google CDN but it is possible to host the jquery files locally.
I do not believe this is a bug in mojoPortal, no one else reports it and I have never seen this error in any of the site I manage.
Hope that helps,
Joe