I did a fresh clean installation of PP 5.3
Here's what happen.
When i go to the admin panel, set the following :
cookie path set as /
cookie name set as XXX (of cos something more readable here)
cookie domain set as .mydomain.com
SMF leave as blank
it will logs me out after i save.
After this i find myself not able to login via the frontend nor the admin. Same for all my users.