Problem: When an administrator, instructor, or student selects "Discussions" from the navbar of a particular course site, a 500 error is triggered.
Solution: An incorrectly formatted HTML link in one of the forum descriptions caused the error to be displayed on the page. Top-tier support administrator removed the bad code from the link, and now the Discussions page loads normally.
Pending: We've asked whether there is a link that might enable a local Site Administrator to resolve the issue without higher-level support intervention.