ACT is a complex CRM and as such it will periodically have issues with the backend database, SQL. There is a great knowledgebase article which lists the most recent DBFIXERS - little small executibles that will correct the issue once it's been found. These are great because often you just run the DBFIXER on the database and don't have to update all computers. Love it when it's that simple.
Here's the link to the article:
http://kb.swiftpage.com/app/answers/detail/a_id/24753/kw/24753