There are many reasons this can happen. Here are some steps to try
1. Make sure you added the Act! License to ACP. Even its it’s a trial, a license may be required.
2. Use the Forgot My Password option on www.MyActLogin.com to reset/resync your Act! Password.
3. In ACP, Go to the servers for the database, then “Enable always connect” and “Enable debug mode”. Now try to login. You should see an Error message
4. In ACP, Go to the servers for the database, Then Click Clean App Pool on the server that is in question.