Calendar events are disappearing when modifying them using a login other than uid.

Example:

LDAP:
uid = pop215776
mail = lawrence.tester@uni-konstanz.de
loginalias = lawrence.tester

SOGoUserSources:

sogod SOGoUserSources '(
{
CNFieldName = cn;
IDFieldName = uid;
UIDFieldName = uid;
baseDN = "ou=people,o=Universitaet Konstanz,c=DE";
bindFields = (
uid,
loginalias,
mail
);
canAuthenticate = YES;
displayName = "Univ. Konstanz";
hostname = "maroua.dmz";
id = unikn_users;
isAddressBook = YES;
port = 389;
type = ldap;
}
)'

Calendar entries done with "pop215776" appear, and the URL for the calendar web frontend is:

https://<sogo-server-address>/SOGo/so/pop215776/Calendar/view

Doing the same SOGo web login with "lawrence.tester" yields:

https://<sogo-server-address>/SOGo/so/lawrence.tester/Calendar/view

All entries do appear normally. But when editing an entry/appointment and saving it it vanishes. It is gone - forever.