Hi Pascal,
This will be a great problem solving exercise for you 🙂
If there are no constraints on that bone and it’s only affected by its parent – take a look at the parent.
Check if there is any accidental transformation on the bone (pose mode), and also check the constraints (toggle them on/off to see if it changes the orientation)
Then continue up the hierarchy hunting for the cause.
This might seem annoying but this will build your skills and your understanding really quickly.
Of course, if you’re still stuck you can upload your file as an exercise and I’ll give you a hand in fixing it.
