import logging
from threading import Thread

class Dummy(Thread):
    def __init__(self):
        super(Dummy, self).__init__()

    def run(self):
        logging.info('Hello!') # Вылетает здесь

def main():
    logging.basicConfig(level=logging.DEBUG,
                        format='%(asctime)s %(levelname)s %(message)s',
                        filename='log.txt',
                        filemode='w')

    Dummy().start()

if __name__ == '__main__':
    main()

Traceback (most recent call last):
  File "D:\Developing\Platforms\jython252rc2\Lib\logging\__init__.py", line 750, in emit
    self.stream.write(fs % msg)
  File "D:\Developing\Platforms\jython252rc2\Lib\logging\__init__.py", line 750, in emit
    self.stream.write(fs % msg)
ValueError: I/O operation on closed file