The source asset generated a file containing no subtitle events. See additional details in the error message for possible root cause.This usually happens for EBU STL files in which the Extension Block Number (EBN) is not properly set causing additional lines to be added to a single event. When these additional line(s) are marked as "comments" the entire entry is ignored, resulting in a timed text file with 0 events. Similar issues can affect other Timed Text formats.
Please check the file. Ensure that the events have not been flagged as "comments" which will be ignored and redeliver.