There is an error showing on top always,
[Joomla Fix] JUser: :_load: Unable to load user with ID
In order to fix it, you need to follow those steps :
1. Open your Joomla database using PHPMyadmin tool.
2. Open "jos_users" table,
3. Find record with "Super User" name and click on edit icon,
4. Change Super User id value to 42 and click on "Go" button,
5. Open "jos_user_usergroup_map" table.
6. Find the record with "group_id" value 8, change "user_id" value to 42, then click on "Go" button,
7. Logout and login again.
I ve checked more on this issue and it is related to K2 items and comments. Original demo items are created with userid 42 and when this id is changed K2 is still looking for UserID42.
What you can do is delete the K2 items and Comments or you need to change userID ( created by ID ) from K2 to match your current userID. You can run below SQL command in phpmyadmin which will change it for you.
Please change following to match your database.
Jos = Your database prefix.
100 = Your new ID which you can see in joomla users.
Run each sql separately please.
UPDATE `jos_k2_items` SET `created_by` = '100',
`modified_by` = '100' WHERE `jos_k2_items`.`created_by` =42
UPDATE `jos_k2_comments` SET `userID` = '100' WHERE `jos_k2_comments`.`userID` =42;