Due to a lot of requests about this pretty common problem.
Steps you did:
- You browsed for a web part:
- You selected one and hit “Add”:
- You got the error message: “An unexpected error occurred.”
This is a know issue with WSS 2.0 and a hot fix  can be requested at Microsoft which corrects this behavior.
I tried this with various customers and we could then successfully add new web parts after applying it.