I am trying to set the label of an item dynamically using the value of an other hidden item in the same page.
I created a computation for the hidden item in the page
"Value protected" only shows for hidden items. This error shows also when you update a visible item that is read only. At least in rel 19.2. See this question also: Can't disable session state protection on Oracle APEX 18.1.0.00.45 for Dynamic Actions updating Page Items
For your hidden item P613_EMPLOYEE_TYPE_LABEL
set the Value Protected attribute to NO.
When Value Protected
of an hidden item is set to YES a checksum is generated when your page is loaded. When you submit the page with a different value the checksum is not valid anymore and you get the error.