Webmin login modal problems

CentOS 8.5.2111
Webmin 1.982
Usermin 1.823
Virtualmin 6.17 Pro
Cloudmin 9.5
Authentic theme 19.84.5

When accessing our Webmin/Virtualmin/Cloudmin server (https://servername.com:10000) we are presented with the login modal as expected.

We enter the login… enter the password (correctly) and authentication fails. When it fails we are presented with the modal requesting, once again, the login.

No matter what we enter it is not accepted. Instead we are requested again and again to enter the login.
This happens often but not 100% of the time.

Possibly related: The Webmin auth modal looks different.
Missing password field
Our other system has login and password fields presented at the same time. This system functions correctly.
Also all of the Usermin auth modals on both systems have both login and password presented at the same time and they work correctly as well.

Webmin auth that functions correctly

I have confirmed that the PAM configuration is identical on the two systems.

Andrew