The problem is related to the Schema password expiring in Oracle. Attached is a pptx that shows how to change the password and set it to never expire.
Okay, I have managed the problem - one of my sqlplus accounts was still expired, but you can list all accounts with the following command:
select username,exiry_date,account_status from dba_users;
Than you can repair the exired account like in the slution above - maybe it´s helpful for someone who is not that familiar with sqlplus.