The ACT! contact management application is a pig. There, I've said it. It's one of those programs that keeps changing owners - that's how much of a pig it is. Seems no-one really wants it.
They re-wrote it in .NET a couple of versions ago, and it ought to be better, but it isn't. ACT! throws "Object reference: Not set to an instance of an object." (a reasonably common .NET error) whenever it feels like it.
In my particular case, I could open it fine when using a Citrix desktop session, but as a published application, it would die all the time.
I tracked the problem to the seamless window. When you run seamlessly, you get Citrix's WFSHELL.EXE running, and not EXPLORER.EXE. You can force a published application to run at a certain screen size, which runs as if you were in a desktop. Hopefully the ACT! forums will give me a better answer.