The operation can't be completed because you don't have permission to access some of the files

I recently installed OMV 5 on my RPi 4b and got everything working...well, almost everything.

I created one user so far, which has all permissions to access the smb/cifs share. When I try to copy small folders (ca. 4 GB) from my Mac (macOS 11 Big Sur), it works fine. But when I try to copy larger folders, I get this weird error:

When I split the large folder into multiple small folders and copy those, it works.

I don't understand why it works with small folders but not with large ones.