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It seems that the custom endpoint you are trying to change has also been scheduled for one-time execution. When you are trying to save the custom endpoint code, the request to the server also passes the scheduled time for execution for this custom endpoint and since this execution time is in the past a Validation Error is thrown in the console.
I have logged this for fixing in our backlog, but until then to solve the issue, please go to Console -> Scheduled Code and delete the scheduled execution for this custom endpoint (you may also change the time so that it is greater than the current date&time). Then you would be able to save the custom endpoint just fine.
Please let me know if this has solved the issue.
I was trying the custom end point feature I've added sample code for triggering a push notification only to not be able to save the code. It gives a validation error but the error is not displayed anywhere.