I spend quite some time to solve it using my PowerDVD15 Ultra software.
I am using the amazing Acer X34 (21:9) with 3440x1440px.
The simple result is: PowerDVD15 is the wrong choice for this setup.
Removing the black bars (which are part of the BR video) is only possible with VLC player and the famous tool starting with Any...
Even though I understand why this problem exists technically I am somewhat disappointed by PowerDVD as I thought 21:9 isnt too new and niche.
In the end the effort is totally worth it - amazing experience watching my BRs in 21:9!
I hope it becomes more famous at some point...
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PowerDVD 18 Ultra and Acer X34 21:9 3440x1440px Monitor