cant log in to admin area using Apple Safari browser after database upgrade to 2.6, log in page simply repeats itself, requesting new password does not change problem.
Check redirect address line after pressing login for garbled address %&A etc.
Try using another browser such as Camino to do the first log in to admin area after the file upgrade and the database upgrade.
Once you have successfully logged in using another browser try a few admin pages, then quit and restart Safari before trying it again to clear its caches, Safari should now log in ok.
I had this on two of my sites and this worked on both as a workaround going from 2.5.1 to 2.6, another site going from 2.6 RC1 worked fine without any problems.