Bug in Mailman version 2.1.5

We're sorry, we hit a bug!

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Traceback:

Traceback (most recent call last):
  File "/home/mailman/scripts/driver", line 80, in run_main
    pkg = __import__('Mailman.Cgi', globals(), locals(), [scriptname])
  File "/home/mailman/Mailman/Cgi/admin.py", line 38, in 
    from Mailman import MailList
  File "/home/mailman/Mailman/MailList.py", line 49, in 
    from Mailman.Archiver import Archiver
  File "/home/mailman/Mailman/Archiver/__init__.py", line 17, in 
    from Archiver import *
  File "/home/mailman/Mailman/Archiver/Archiver.py", line 31, in 
    from Mailman import Mailbox
  File "/home/mailman/Mailman/Mailbox.py", line 21, in 
    import mailbox
  File "/usr/lib64/python2.6/mailbox.py", line 18, in 
    import email.message
ImportError: No module named message

Python information:

Variable	Value
sys.version	2.6.6 (r266:84292, Jun 20 2019, 14:14:55) [GCC 4.4.7 20120313 (Red Hat 4.4.7-23)]
sys.executable	/usr/bin/python
sys.prefix	/usr
sys.exec_prefix	/usr
sys.path	/usr
sys.platform	linux2

Environment variables:

Variable	Value
SERVER_SOFTWARE	Apache/2.2.15 (CentOS)
SCRIPT_NAME	/mailman/admin
SERVER_SIGNATURE	Apache/2.2.15 (CentOS) Server at mailman.connactivity.com Port 80
REQUEST_METHOD	GET
SERVER_PROTOCOL	HTTP/1.1
QUERY_STRING
HTTP_USER_AGENT	CCBot/2.0 (https://commoncrawl.org/faq/)
HTTP_CONNECTION	Keep-Alive
SERVER_NAME	mailman.connactivity.com
REMOTE_ADDR	100.26.179.196
SERVER_PORT	80
SERVER_ADDR	149.56.254.165
DOCUMENT_ROOT	/home/mailman/public_html
PYTHONPATH	/home/mailman
SCRIPT_FILENAME	/home/mailman/cgi-bin/admin
SERVER_ADMIN	support@connactivity.com
HTTP_HOST	mailman.connactivity.com
REQUEST_URI	/mailman/admin
HTTP_ACCEPT	text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
GATEWAY_INTERFACE	CGI/1.1
REMOTE_PORT	56522
HTTP_ACCEPT_LANGUAGE	en-US,en;q=0.5
HTTP_ACCEPT_ENCODING	br,gzip