Started by upgrading site from 5.3.2 to 5.4.0.
As my site is in russian, I received the error 500 right after the "successfull" upgrade. To reanimate my site, I went to the Database and cloned the line in PortalLocalization table, but changed in second line language from ru-RU to en-US.
Today, I upgraded my site 5.4.0 to 5.4.1. As I can see, the portal's localization problem persists and was not resolved.
The other issue that I have is in the Control Panel. Go to: Admin>Site Settings>Appearance. Now try to change the LOGO, by selecting a different folder from DropDown list. It will cause a PostBack, and the tree menu will collapse.
Now, in Admin>Site Settings>Advanced Settings, click on Host Settings. The page is posted back. All the tree menu collapses once again.
Nice work.
I don't know if there are any other issues somewhere, but again, it's not perfect.
Oh, yes, error on Logout.