Post preview gives page not found error message

I am having trouble with two preview functions in WordPress. First, when I try to preview a post, I get a “page not found” message. Second, when I try to preview a page that I have previously published, I get a “You do not have permission to preview drafts” message.

I’ve gone through many of the earlier fora about post preview not working and I have not been able to figure out the problem.

Can you help?

thanks in advance