Same problem. Problem appears to be related to Safari. I loaded Google Chrome for Mac on my iMac and was able to change the password with no problem.
This just happened and hasn’t been a problem in the past, so either a recent Safari update or a change in the DoD program created an incompatability. May be related to pop-ups or Safari’s password autofill.
DoD support suggested clearing all history and cookies which might have worked but would be painful. This could be a long term problem since DoD requires password change every 60 days.