A parent cannot register as it says the DOB for their child is incorrect?
The first thing to check here is the DOB that you hold for the child in ParentMail. You can quickly check this by going to Users > List, then search the name of the student in question (when a parent is registering it will always ask them for the DOB of their eldest child). If the parent confirms that the DOB you hold is correct, the next suggestion would be for the parent to attempt the registration link on another device to see if the issue persists. E.g: If attempting on a mobile device, can it be done on a PC and vice versa.