Solved Subversion error: MKCOL of '...': 405 Method Not Allowed

I hope this helps some other poor person before they tear all their hair out.

When i commit using VisualSVN i get error message

svn: MKCOL of '...': 405 Method Not Allowed

What?! What does that mean?

The directory already existed in the Subversion repository.

Ultimately, the error was therefore mine - trying to add a directory existed in source control. However, a more descriptive error messsage would have been nice!