Yes, I found a temporary solution. Due to the fact that messages from that workflow seem to arrive on the n8n discord channel, the issue can’t be located on the discord side.
As Navigating the editor UI | n8n Docs states that the course is based on n8n 1.82.1, I switched back my n8n instance to exactly that version.
I had to build the workflow again, because some of the nodes from the newer version I had before wouldn’t work.
With that version, the discord node worked as expected and I was able to finish the workflow.
So the problem resides in the discord node of newer n8n versions - but I can’t tell in which version the issue occurred for the first time.