Guessing as there are no real details here... do you have any custom fields that are DATE format? If so, you will need to open those fields and re-save them as there is a new parameter on date fields added in 2.6.3 for the date format. Without that being saved, you will have an issue. If that isn't it, log a support ticket.
Is this by registration for individual and group registration?
I have same problem. If i use the checkbox it returns to eventpage.
If i click on the big buttons it continues to registrationform.
This problem only occurs by firefox and not IE