What does this mean?
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We had this issue as well, and it seems we identified the cause.
What happened was that an underlying expression rule was changed, and instead of returning a CDT , it returned a map that contained the CDT in the .data attribute. The data output Target was of that CDT type (not a Map). So there was a mismatch between the process variable data type and the result of the expression in the data output.This issue only occurs when the above happens in the Data Outputs of a script task.
If this is done in the Data Inputs, there will be no such error, and I assume the process variable will remain empty.However, this broke any chaining in the process that passess through this script task. (which was another issue we faced)
So the solution in our case: check the types of the expressions in the data output tab, and make sure they match the process variable