When deploying a new web application, the Manager app states quite clearly that the Context Path field is 'Optional'. It is however /required/ and the deployment will fail unless a valid one is provided. The documentation even states that at a minimum this has to be a single '/' unless specifying the ROOT application. Furthermore, the very succinct and unhelpful error message mentions nothing regarding this expected format.
Correction, the documentation states that it must start with a '/', and that it must be provided unless specifying the ROOT application. However it still fails unless you specify one, when when you intend to reference the root app
This was fixed in 6.0.19 onwards. *** This bug has been marked as a duplicate of bug 46115 ***