In mots countries and in Iran flight plan and DC mean getting large overflying fees .
No flight plan = difficult to get the money.
No or Expired PLN = no info to ATC on first call.
So, probably seen the dep airport, the flight was refused, Crew declared they could not return and an offer to land in Iran to sort out the " problem" was offerred and accepted by the crew,
Not an abnormal procedure .
A non-event.