This was probably caused because you took your members page out of the site map, or moved it somewhere else besides under home. If you have it under another page move it below home, and than add custom attribute exclude from nav to keep it out of your page nav. If you deleted it or it is not there create a page under home called members, and than add exclude from nav. This should fix all the issues your having. If not than your form must be pointing somewhere other than where it should.