When using nested workflows, a child workflow tries to update an inherited variable with a new value.
In the nested foreach workflow, the Publish Value is originally saved (only possible when using the JavaUI) in lower case letters.
In the AWI the Publish Value is always shown in upper case letters.
Screenshot from the Java UI:
<Please see attached file for image>

Screenshot from the AWI:
<Please see attached file for image>

So, for the same RunID you have two values of the variable &item#, one in upper case letters and one in lower case letters, which leads to a unique constraint violation.