Recently I've experienced problems dealing with some long feeds which exceeded the VARCHAR 255 limitation of the field 'link' of aggregation_item table.
I've changed this datatype for TEXT and i have it working.
Another solution would be changing to VARCHAR(65535), but this is only available in MySQL 5.0.3 and older. (http://dev.mysql.com/doc/refman/5.0/en/char.html)
I suggest to change this data type for avoiding limitation issues