For some reason, the upgrade process created new entries in the accounts table for the Administrator and the Default Manager, but did not successfully finish updating all the relevant keys.
In fact it didn't seem to update any of them: the account_preference_assoc table in particular still referred exclusively to account_id values of 1 and 2 (the original accounts for Administrator and Default Manager). There were no preferences set up at all for the new accounts.
However, the admin_account_id value in the application_variable table was set to the value for the new entry in the accounts table.
As soon as I set this value back to 1, all the problems disappeared.
The problems we were seeing were that statistics did not show, that in My Account the language was set to Albanian, that User Interface preferences had no columns specified to show any statistics (which is why none were showing), and that no changes to any of the My Account settings took effect (even though the system reported that the changes had been saved, and no error messages appeared anywhere in the debug log).
andrewtblake got a reaction from andrewatfornax in Changes To My Account Are Not Saved