Hi Mark,
Many thanks for your reply.
Is now solved as follows:
Today I found on the list the new 'date/time' field "birthday", but not the 'text area' fields. So I deleted the 'text area' fields and created new 'text input' fields, which are now on the list.
PS: about the issue Rename the certificate output file, is there a way to add some variable, for ex. student name?