Your .Mac account has nothing todo with it, it doesn't affect iWeb in that way. Usually if iWeb refuses i'm my case it has to 2 things:
- The TemplatesInfo.plist file is corrupt/it's been corrupted. If you know what i'm talking about, you probably have been playing with it, review it for errors or replace it with a backup copy. If you don't know what i'm talking about it probably isn't the problem. In any case, reinstalling iWeb will solve this problem.
- The Domain file has been corrupted. One of the problems (IMHO) ofiWeb is that it keeps all your pages and sites in one big XML file, if there is a problem saving one of your pages it can corrupt this file, not allowing iWeb to load, this has happened to me twice, but this isn't common. This file is located under:
User>Library>Application Support>iWeb>Domain
I would recommend you to reinstall iWeb, this won't destroy anything you have been working on and is the best way to solve the problem. If you still have a problem, depending on how much you have done with iWeb, i would recommend you to delete the Domain file (see path above), reinstall and start again.
I have had this 2 problems and that is how i solved, still it can be something else.