Sometimes, you may see an error message that says just that, "Error" -- nothing else. It typically clears up on its own, perhaps after doing something else.
This error is mainly caused by cookies being blocked or lost. Once you login, your session info is held in a cookie. If you arrive on a page that the Course Manager thinks you shouldn't have access to, your session is lost and you see the generic error.
You may have been logged in on a different class that uses the legacy LTI domain, or you may have a session cookie open from another domain (such as your LMS, which may be framing the Course Manager) that blocks you from also setting the Course Manager cookie.
One way to help avoid this is to make sure your browser allows cross-site cookies (may be called cross-site tracking or scripting). The Safari browser (default on Mac and iOS devices) is the worst offender as they continue to increase their cross site-tracking prevention.
If you have are unable to overcome this issue by fixing your settings, you can also try clearing your browser's cache/cookies or by using an incognito (private) browsing window. You can also try switching to a different browser for a little while as a workaround. But most of the time, this will eventually resolve on its own after what seems like random clicking around or just coming back later.
Feel free to reach out to NROC for technical support if you have ongoing difficulty with this: https://nrocnetwork.org/ticket