You'll need to check the User assigned to the "Default Lead Creator" or the "Automated Case User" and review the Profile assigned to them.
Users that have "Login IP Ranges" defined on their Profile and also the Profile permission "IP Restrict Requests" checked will see delays or errors when generating the requests.
Please note that the "IP Restrict Requests" permission is intended to work on Portal Users in order to enforce them to only login from a limit of 5 IP addresses. It is an extra security measure that doesn't need to be checked for a regular (full license) User.
To resolve this issue you should uncheck the "IP Restrict Requests" Profile permission.
Q: Does removing this permission impact the IP Ranges I've defined for the Profile?
A: No. The login IP Ranges defined in the Profile are still valid. You don't require the "IP Restrict Requests" permission to set the Login IP Ranges.
Q: Why are the Lead / Case created after 24 hours?
A: The initial failure in the submission is bypassed by the queue mechanism that governs Web-to-X Requests. The request is enqueued and retried 24 hours later. The initial request hits the validation but, once it's enqueued, it won't hit it again. That may cause that you see Cases / Leads created 24 hours after their original submission time.
Q: Does the "IP Restrict Requests" permission affect other requests?
A: Yes. Other requests like Email to Case, Email to Salesforce, Salesforce for Outlook, Mobile login or API access using the User context could be impacted if you have "Login IP Ranges" defined plus the "IP Restrict Requests" Profile permission. They will fail with a "554 Transaction failed" message. More about this: 554 Transaction failed: null error with SFO and Email to X