The CSV Import module wants a column named "Geburtstag". The UMC Attribute is named (in german) "Geburtsdatum". This is quite confusing. I think the CSV Import module should accept both notations.
This issue has been filled against UCS@school 4.0. The maintenance with bug and security fixes for UCS@school 4.0 has ended on May 31, 2016. Customers still on UCS 4.0 are encouraged to update to UCS 4.3 (or later). Please contact your partner or Univention for any questions. If this issue still occurs in newer UCS versions, please use "Clone this bug" or simply reopen the issue. In this case please provide detailed information on how this issue is affecting you.