I found no method of of accessing the tt-rss app as admin.
I guess this happens because of SSO and me using my normal unprivileged yunohost-account to access tt-rss.
I worked around this by accessing the mysql database and making myself admin-user:
MariaDB [ttrss]> update ttrss_users set access_level='10' where id = 2;
Is there any other way?