Did you try manually deleting all files in the core/cache directory and clearing your browser cache and cookies?
Yes, i deleted core/cache directory and cleared browser cache, still the same...
and i did not found the lost files it mentioned in the message after login: "sytem/dashboard/widget/feed.
and i did a second upload of all 2.6.0 files and a new update: the same result.
I don't understand why it says Version 2.58pl after update?
Did you use the UpgradeMODX extra to do the upgrade?
I'm glad I could help. Thanks for reporting back.