Mailman archive function fails with an error regarding replace_payload_by_text

Symptoms

When a Mailman archive process is run, an error similar to the following is produced:

[root@server]# sudo -u mailman /usr/local/cpanel/3rdparty/mailman/bin/arch --wipe newlist_cptesting.tld
Pickling archive state into /usr/local/cpanel/3rdparty/mailman/archives/private/newlist_cptesting.tld/pipermail.pck
Traceback (most recent call last):
File "/usr/local/cpanel/3rdparty/mailman/bin/arch", line 202, in <module>
main()
File "/usr/local/cpanel/3rdparty/mailman/bin/arch", line 190, in main
archiver.processUnixMailbox(fp, start, end)
File "/usr/local/cpanel/3rdparty/mailman/Mailman/Archiver/pipermail.py", line 554, in processUnixMailbox
mbox_iterator = iter(mbox.values())
File "/usr/lib64/python3.6/mailbox.py", line 119, in values
return list(self.itervalues())
File "/usr/lib64/python3.6/mailbox.py", line 109, in itervalues
value = self[key]
File "/usr/lib64/python3.6/mailbox.py", line 76, in __getitem__
return self._factory(file)
File "/usr/local/cpanel/3rdparty/mailman/Mailman/Mailbox.py", line 88, in scrubber
return mailbox.scrub(msg)
File "/usr/local/cpanel/3rdparty/mailman/Mailman/Mailbox.py", line 110, in scrub
return self._scrubber(self._mlist, msg)
File "/usr/local/cpanel/3rdparty/mailman/Mailman/Handlers/Scrubber.py", line 408, in process
replace_payload_by_text(msg, sep.join(text), charset)
TypeError: sequence item 0: expected str instance, bytes found

Description

Mailman is failing when archives are built for mailing lists that contain certain data. This results in an error mentioning replace_payload_by_text.

We've opened an internal case for our development team to investigate this further. For reference, the case number is CPANEL-47266. Follow this article to receive an email notification when a solution is published in the product. 

Workaround

None available at this time.