Affected course link:

https://www.outsystems.com/training/courses/71/Style+Guide+Architectures/?LearningPathId=17

While the course page itself is still active, starting the course or manually selecting any of the 3 lessons of the course results in a "Lesson not found!" error message and a redirect to the Courses home page.
Interestingly, the quiz page at the end of the course is still available.

This doesn't seem worth a support ticket, and it's not necessarily a question or difficulty, so a discussion post seemed most appropriate for the topic.