There is some sort of SVN error which doesn't allow one to use spaces in ones directory structure when using the "externals" svn property.
The solution to this one is quite short and simple:
Instead of using spaces, use %20 which the svn does understand as a space character.
So if your external consists of something like this:
rewrite it to:
Yeah, i know, ugly... but it works.
BTW - Maybe this isn't a general SVN problem, it might just be a TortoiseSVN problem.