I had to change the server name of my mailman server. I changed it in /etc/mailman/ as follows:
# Default domain for email addresses of newly created MLs
# Default host for web interface of newly created MLs

BUT! Some links in the mailman site were OK (new) and others were not OK(Old servername)

To remedy to this all the mailing list need to be internally modified to reflect the new hostmname in the sites URLs and the emails URLs.
Simply run the following 2 commands:
cd /usr/lib/mailman/bin/
./withlist -l -a -r fix_url -- -v

This runs withlist and tells it to lock the lists (-l) process all lists (-a) process by calling fix_url in the module with arguments of the list instance and -v which causes fix_url to report what it’s doing. The — is to separate the -v option for fix_url from the withlist options since there’s no listname to do that in this case.

For mailing lists with different URL then the site is suggesting the following:
If you have more than one virtual host, you have to process the lists
one at a time with

bin/withlist -l -r fix_url listname -u url_host

but you could wrap that in a shell script to run the command repeatedly
for all the listname/url_host pairs.