At this page the example for the Portal page contents looks like this:

<form method="post" action="$PORTAL_ACTION$">
   <input name="auth_user" type="text">
   <input name="auth_pass" type="password">
   <input name="redirurl" type="hidden" value="$PORTAL_REDIRURL$">
   <input name="accept" type="submit" value="Continue">
</form>

This will work fine with Firefox but won't work with IE if you submit the form by hitting enter (not
clicking button).
I have to admit that we changed some more things here as we did hide the passwordfield, set a static
password and generate usernames as ticket-IDs that are checked against an external Radius server.

However, to make this IE compliant when just hitting enter it should look like:

<form method="post" action="$PORTAL_ACTION$">
   <input name="auth_user" type="text">
   <input name="auth_pass" type="password">
   <input name="redirurl" type="hidden" value="$PORTAL_REDIRURL$">
   <button name="accept" value="ok" type="submit">Continue</button>
</form>

This works with Firefox and IE, even when hideing additional fields. If you don't consider this as
valuable enough to change the content of this page it hopefully will at last help somebody with the
same problem I had.

Holger