This error occurs when you are using Internet Explorer 8, which is not supported as of ACT for Web version 11.1.
You may use the work-around of "Compatibility view" in IE 8 to get around this error.
In Internet Explorer 8, go to Tools > Compatibility View. Be sure Compatibility View is checked. Go to Tools > Compatibilty View Settings:
Add "myactdb.com" to the website addresses: