you'll need to query "isys_catg_relation_list" (this is the "relation" category) and inside that table you'll need to select the rows where the field "isys_catg_relation_list__isys_relation_type__id" reference to the isys_relation_type rows with the types "C__RELATION_TYPE__USER" or "C__RELATION_TYPE__ADMIN".
It should look something like this:
FROM isys_obj AS obj_main
LEFT JOIN isys_catg_relation_list AS j4 ON j4.isys_catg_relation_list__isys_obj__id = obj_main.isys_obj__id
LEFT JOIN isys_relation_type AS jn3 ON jn3.isys_relation_type__id = j4.isys_catg_relation_list__isys_relation_type__id
WHERE jn3.isys_relation_type__const = 'C__RELATION_TYPE__ADMIN'
OR jn3.isys_relation_type__const = 'C__RELATION_TYPE__USER';
is it possible that you are using the OPEN i-doit version and work with multiple mandators? The first problem occurs, if one mandator has installed the LDAP module, but the second doesn't - You can re-install a module during a i-doit update.
For the second error you could try to update the necessary table field to allow "empty" values.
as you mentioned, you're using a hosted web space so most probably there exists Web-GUI to manage services like MySQL. In the previous steps installer asked root user credentials for the MySQL database in order to create dedicated i-doit CMDB user.
If you have a possibility to maintain your database users over Web-GUI, try to create dedicated i-doit CMDB user manually and start your installation from the beginning.
You could add your suppliers as a organisations (in the contacts) and add these as "contact assignment" with the role "supplier"
You could create an own category in the administration area: "Administration > Custom categories".