When calling isValid in the superboxselect component firebug reports a 'too much recursion' error.
It can be fixed by removing the clearInvalid calls in validateValue.
ExtJS itself doesn't call clearInvalid in validateValue either (for example in Ext.form.NumberField), so I assume this is the correct fix for the problem.
If not please correct me.

Code:
validateValue: function(val){
        if(this.items.getCount() === 0){
             if(this.allowBlank){
                 /*this.clearInvalid();*/
                 return true;
             }else{
                 this.markInvalid(this.blankText);
                 return false;
             }
        }
        
        /*this.clearInvalid();*/
        return true;
    },