hitting the 'alter channel subscriptions' link on a system details page in some
cases results in an ISE. Traceback linked to in above private comment.
Encountered in prod with a proxy setup.
May be related to bug 248876.
note: a workaround is to go to the channel you'd like to subscribe the system
to, and click on the 'target systems' tab. you should be able to subscribe the
system that way.