What do the logs show? Try to create a ticket as a customer, note the time and then look into /var/log/zammad/production.log for this time. Is there anything?
Search for the error ID your user gives you and you’ll find the reason.
These IDs are not generic but unique to the situation so nobody can help you at this point.
Thanks all, appears it was because of an issue with a multiselect object i could not get working. This was clear as the issue when reviewing the logs - Removed the object and now i customers can submit tickets.