can't login to list admin interface

As of a few days ago I can't login to the
administrative interface for any of my lists. This is
not browser related (I have tried with several browsers
and have checked cookie settings).

When i enter a password into the administrative login
page the page just reloads. There is no error message
displayed.

Mailman version is 2.1.5 running on Debian, with Python
2.2.1

I have checked the log files and there are no relevant
error messages.

Apache shows a log of the POST request:

x.x.x.x - - [16/Feb/2005:09:47:57 +1100] "POST
/cgi-bin/mailman/admin/soccer-uni HTTP/1.1" 200 2281

I have found that if i manually enter a GET request I
can login. e.g.

http://host//cgi-bin/mailman/admin/soccer-uni?adminpw=mypassword

However when I try and change any of the settings the
page just reloads without the changes. So it looks to
be like a problem with processing POST requests.