A subflow results in error state. If you check the logs it records a nullpointer exception:
Flow Designer: Operation ... failed with error: java.lang.NullPointerException
It fails when the subflow is finished and the output is passed back to the main flow. It loops through all the outputs, if it can't process it this error will be thrown.
You need to check the outputs of the subflow and see if there are any problems here. For example, an ouput could have type Records but not actually reference a record. This would result in this error.