*** This bug is a duplicate of bug 1602608 *** https://bugs.launchpad.net/bugs/1602608 In the traceback you posted, the exception is raised by cgidata.keys() and is saying that cgidata has no list of keys, not even an empty list. cgidata is the result of cgi.FieldStorage(keep_blank_values=1) where cgi is the Python cgi module. I think this is the result of the web browser or web server posting the data with a bad Content-Type. Current Mailman (2.1.15 is almost 10 years old) and this issue is a duplicate of https://bugs.launchpad.net/mailman/+bug/1602608 which see, particularly https://bugs.launchpad.net/mailman/+bug/1602608/comments/4 ** This bug has been marked a duplicate of bug 1602608 mailman crash for subscription in webinterface -- You received this bug notification because you are a member of Mailman Coders, which is subscribed to GNU Mailman. https://bugs.launchpad.net/bugs/1974478 Title: Mailman 2.1.15 found a bug To manage notifications about this bug go to: https://bugs.launchpad.net/mailman/+bug/1974478/+subscriptions