A workaround for when this error occurs is this:
(In the directory for the domain i.e. domains/qa)
bin/stopserv
rm -r generated/*
bin/startserv
Log in again to the admin server and everything works. At least, it works for a while until the error occurs again.
[Message sent by forum member 'davideagen' (davideagen)]
http://forums.java.net/jive/thread.jspa?messageID=306980