OK, I found the issue meanwhile. There is a very tiny "+" sign near the "Person" settings for a contact. If that is expanded one can insert login credentials and set the group membership.

I've problems organizing new users. AFAIK and read in the Wiki users will be created in "Contacts" and can be assigned to the existing groups. Additionally I should be able to set a username and password for contacts to be able to login.
I'm logged in as admin into my instance and can create contacts, groups etc. But I neither have the option to set username/password for contacts, nor am I able to assign them to groups.
I have the LDAP module installed but there is no active LDAP server configured. Could the mere existence of the LDAP module be the problem?

ah yes. Thanks! That seems to be the problem. My MySQL DB was set to strict mode….

I've installed i-doit version v0.9.9-3 in conjunction with a MySQL database. After successful installation I tried to configure an LDAP server with authentication. But when I'm trying to add a new LDAP server, filling out all the available fields I get the following error message:

Error! Could not save!
isys_exception_database : Query error: 'UPDATE isys_ldap SET isys_ldap__isys_ldap_directory__id = '1', isys_ldap__hostname = 'my_LDAP_server', isys_ldap__port = '389', isys_ldap__dn = 'my_LDAP_DN', isys_ldap__password = 'my_LDAP_password', isys_ldap__user_search = 'o=nokia', isys_ldap__group_search = '', isys_ldap__filter = '(objectClass=user)', isys_ldap__active = '1', isys_ldap__recursive = '1', isys_ldap__timelimit = '15', isys_ldap__tls = '0', isys_ldap__version = '3', isys_ldap__status = '' WHERE (isys_ldap__id = '1');': Incorrect integer value: '' for column 'isys_ldap__status' at row 1 

I can't see the LDAP Status field in the HTML form. So how can I correct this?