Symptoms
We saw the following error when hitting a line rule in the beginning stages of the workflow: "A list can only be used in an expression if it contains exactly one value".
The line rules were based on a smartobject item reference. Even after removing the line rules and re-adding them again, the issue still persisted.
Diagnoses
This was due to a broken SmartObject item references. We added a couple of SmartObject item reference fields to a mail event for testing, and the fields were all empty.
Resolution
After we re-created the smartobject item reference, it started to return values again and we were able to get past the line rules that threw the error.