DB2 Linux authentication fails

I have DB2 Express-C v10.5 instance configured to authenticate against LDAP. The LDAP sever is going to be shutdown and I should configure the same DB2 instance to use Linux authentication.

I copied users from the LDAPĀ  server to a local Linux host running DB2. Then I did shutdown the LDAP server. After that I changed DB2 authentication settings db2 update dbm cfg using SRVCON_PW_PLUGIN IBMOSauthserver (used to be IBMLDAPauthserver before) and restarted DB2.

Applications access the database with the username db2smth (name changed due to privacy reasons). I can connect to a database with db2 connect to dbname user db2inst1 using '********' but connecting to the same database as db2smth fails:

db2 => connect to dbname user db2smth using '********'  SQL30082N Security processing failed with reason "24" ("USERNAME AND/OR PASSWORD INVALID"). SQLSTATE=08001 

su - db2smth and su - db2inst1 works fine which means that Linux authentication works fine.

How can I diagnose what’s wrong with the authentication?