We can see the user name shows 'undefined' after the upgrade.

Document ID : KB000103492
Last Modified Date : 27/06/2018
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After the upgrade, we can see the the user name shows 'undefined' on upper right.
Please let me know the root cause and solution for this.

User-added image
This happens when the browser cache does not have correct one. 
If you clear the cache then you will be able to resolve this.