Huh, it is odd that NamedTemporaryFile got used without using an explicit name. Let me take a look a bit more, but in concept I'll get this merged in a day or two.

Got this problem too.
https://stackoverflow.com/questions/66744497/python-tempfile-namedtemporaryfile-cant-use-generated-tempfile

The following adjustments also solved the problem:

def _save_virtual_workbook(self, wb):
    try:
        tmp = NamedTemporaryFile(delete=False)
        save_workbook(wb, tmp.name)
        tmp.seek(0)
        virtual_workbook = tmp.read()
    finally:
        tmp.close()
        os.unlink(tmp.name)

    return virtual_workbook

Sorry for the delay, just back from vacation. I'll get a version published to PyPI today. @rdmolony @AlexKovyazin