When building Cesium from source code, the build-sandcastle command fails with an error.
This was also reported at The build-sandcastle command fails during build · Issue #12866 · CesiumGS/cesium · GitHub
There clearly is an issue with Windows Path-vs-URL handling here. I added a comment in the issue that contains something that might be a workaround for the immediate issue. A proper solution still has to be investigated.