eventosfera,
Sorry to hear you’re having trouble here, but hopefully I can help. I was just able to successfully submit this test event without any issues: “Test Event: Modern Tribe Support.” (Please delete that event)
I would follow up with your users and have them try to submit the event again. If they’re still unable to, have them record their steps one by one to make sure they’re accessing the correct page and following the appropriate steps.
I hope that helps, but please let me know if you have further questions. Thanks! 🙂
-Casey-