I encountered this problem recently and was not able to find anything on the community so wanted to post my solution.
We recently had someone add a Closed Date to an OU on accident. When we go to remove the date, we get an error: “Invalid Closing date ‘1/1/70’ must be after Opening date”
It seems the system requires a closing date after one has been entered. The only way we were able to fix this was to download the JSON payload for the Org Unit (fields=*), edit it to remove the Closed Date, and import it with an Import Strategy of Merge and Merge Mode = Replace.