Thank you very much! You are right indeed, the atlas was not matched to the T1 MRI but is rather in MNI space, that was a mistake on my part.
Thank you for the tip of not loading a volume atlas on a surface as well. I am currently trying to compute a volume instead, I will get back to you if I run into issues.
EDIT:
I get this error message when trying to load the atlas to the BEM volume