I’m not sure, but I think I tried to clear the cache with the hummingbird plugin. And a 503 error appeared. After that I was not able to login anymore.
The site was having those 503 error since the migration. it worked smooth on production. I also deactivate the events calendar plugin from the dashboard before that and then it crashed because of a function in functions.php that refers to events calendar. But I don’t know if it has something to do with that.