Error message: There's no line here to end

Check for \newline or \\ commands and what comes before. For instance don't write \\\\ to get one further empty line. Possible reasons and solutions are given here in the UK TeX FAQ: No line here to end.

To complement this: sometimes you really wish to break a line but LaTeX doesn't allow it because you are in vertical mode. In such cases \leavevmode directly before \newline or \\ helps. Some people use a quick fix and insert some empty space like in ~\\ to repair that.

I have had exactly the same problem with one difference: the source file wasn't mine, I received it by email. It worked for the sender, but I got the "There's no line here to end" error associated with an \opening{} command in my machine.

Matching the code against a letter class intro in a blog, Geek Thoughts, I found a difference: the lacking of a second argument for \begin{letter}. Once provided, everything worked fine for me.

Hope it helps.

Empty lines above and below the opening caused the error. Removing the empty lines solved it.