I've been using piwigo on my personal site for a while, it's working quite well so far, great sofware!
I've seen one problem with piwigo, though, which confuses users: after a successful password change via the 'Forgot password' link, the main window keeps showing the 'enter your new password' boxes. The left menu does change to reflect login has in fact been successful, but granny-type users definitely won't know what's happening. I too missed it the first time.
This might be similar to http://piwigo.org/forum/viewtopic.php?id=17492 where (IIUC) it was happening after a login attempt.
I'm using piwigo 2.3.3.
Thanks for the time.