I didn't have that problem but the most likely reasons are either they have your information in the system wrong, or it's more a syntax error, meaning the way it's written. The date of birth field only gives you a box to write in, but people write the date in various ways. I'd try various ways and see if any other ways work - when I did mine it was Day/Month/Year, so using today's date that would be 18/7/2012, but try 18-7-2012, 18.7.2012, 18/7/12 or whatever ways you can think of.
If the problems still occur it's probably that your information is wrong in their database, so try calling up the university.