Workflow '&' no. & step no. &: Error in expression '&'

What causes this issue?

The expression could not be evaluated.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

INCLUDE NA_WF_ERROR OBJECT DOKU ID TX

How to fix this error?

Precise information on the type of error can be found in the previous
message in the workflow log.
Correct the error and resume the interrupted workflow. If it is not
possible to make a correction, start the workflow again.
INCLUDE NA_WF-ADMINISTRATOR OBJECT DOKU ID TX

Error message extract from SAP system. Copyright SAP SE.