Diagnosis
Expression &V1& should provide a responsible agent for workflow stepnumber &V2&. However, the result does not have the correct data type orgave errors in the evaluation:
Expression,,&V1&
ABAP type,,&V3&
Length,,&V4&

System Response
If no other valid agent (expression) was specified, the workflow hasbeen stopped and forced into the error status.

Procedure
Correct the expression's definition/implementation so that it providesa value with reference to the ABAP Dictionary structure SWHACTOR(ABAP type: C14). In particular user names cannot be on their own here,only with preceding organizational object type "US...".
Object references are not possible here either.
You can then continue a workflow that may have stopped with theRestart After Error function.