“无法在[路径]初始化App Engine SDK”是什么意思，以及如何解决它？

I'm using the the "Google Plugin for Eclipse", and installed both the "Web Toolkit SDK" and the "App Engine Java SDK" with it. Weirdly, the App Engine SDK can't be initialized while the Web Toolkit SDK works fine. Trying to point Eclipse to the root GAE SDK directory will cause the "Failed to initialize App Engine SDK at [path]" error. Downloading the GAE SDK separately will yield the same result.I'm using the the "Google Plugin for Eclipse",