Update 2018
Starting with Adaxes 2018.1 property display names are managed in the Administration console. For details, have a look at the following help article: https://www.adaxes.com/help/CustomizePropertyDisplayNames.
Original
Hello Thorsten,
You need to use the LDAP name for the property instead of its display name. All Adaxes virtual properties have the adm- prefix in their LDAP names. Thus, the LDAP name for the CustomAttributeDate1 property is adm-CustomAttributeDate1.
<friendlyNameItem>
<ldapName>adm-CustomAttributeDate1</ldapName>
<friendlyName>Start-Date</friendlyName>
</friendlyNameItem>