I have found out that when ever there is a String data type used in the filter section of a portlet, it can not be used as multi select.
Multi select workls fine when the attribute is of Numeric type.
When ever this issue occurs, I have created a lookup with key as the related ID and the display attribute as the NAME. (ie. For making Project Name as a multi select in the filter section of the portlet, I created a lookup with the Key as Project ID and the display attribute as Project Name )
Any idea why this problem occurs? Why is that a String attribute alone gives error?