The recent 22.6 Android Developer Tools (ADT) update broke quite a few things: AVD creation and editing, ProGuard, performance, etc. Rather than revert to 22.3 like many folks, I decided to muddle through with some work-arounds and hold out for the fixes in 22.6.1.
Well, 22.6.1 has arrived and at least fixed the AVD issues, but ProGuard still fails with:
Error: Unable to access jarfile ..\lib\proguard.jar
Fortunately, working around that bug is easy: simply edit proguard.bat and provide an absolute path to the JAR.