That error could be due to a missing jar-file in JAmiga:lib. It should've been installed with the developer's kit, but perhaps it's disappeared between your installs?
Anyhow, do a:
dir jamiga:lib/
If everything is ok, you should hava a JAR-file, ecj-4.3.jar there.
If you don't, then that's what's wrong.
Then, in order to download the dev-kit again, you need to do:
delete JAmiga:javac
If you run AmiUpdate now, you should have the JAmiga developers kit in the list again. And hopefully when you install it again, everything should work.
(If the AmiUpdate for some reason fails, it's most likely due to some file being locked by the JamVM process (yet another bug!). Please copy the "unpacked" dir in ram: to disk, reboot, and copy the "unpacked/JAmiga"-directory content to JAmiga:. Then, perhaps, if all the stars are aligned, it'll work.)
That's what I can come up with at the moment. Once again, thanks for your patience!