In brand new phpBB 3.01 installation:

Links shown in status bar, hovering over buttons such as
UCP - Board Preferences - Submit button
show a destination like:

.ucp.php%3Fi%3D165?i=prefs&mode=personal

This then results in 404 not found error

Workaround of commenting out

$page_name = urlencode(htmlspecialchars($page_name));

from includes/session.php seems to make board functional again -
but is probably far from ideal solution ?

I realise this is quite possibly Server/PHP configuration related,
otherwise would be far more widespread issue.

However, I don't know what configuration would cause this,
and I'm reporting as a possible bug simply because as far as the OP
in topic linked below was concerned, this was a showstopper.

http://www.phpbb.com/community/viewtopic.php?f=46&t=987825
(LONG topic...)