Apparently, box_utils.py isn’t part of a package. You still can import functions defined in this file, but only if the python script that tries to import these functions lives in the same directory as box_utils.py, see this answer.
Nota bene: In my case, I stumbled upon this error with an import statement with one period, like this:
from .foo import foo. This syntax, however, tells Python that foo.py is part of a package, which wasn’t the case. The error disappeared when I removed the period.