"user does not exist " when calling DBMS_LOGMNR.START_LOGMNR
That is what you get when you break and ignore the basic concepts of the CDB architecture and common users and use unsupported &quot;workarounds&quot; (ALTER SESSION set &quot;_ORACLE_SCRIPT&quot;=true).
Above code works flawlessly when you create the user in a supported way.
-- create admin user on CDB
CREATE USER c##myadmin IDENTIFIED BY myadmin DEFAULT TABLESPACE users QUOTA UNLIMITED ON users ACCOUNT UNLOCK
/
-- allow access to all PDBs to the admin user
ALTER USER c##myadmin SET CONTAINER_DATA=ALL CONTAINER=CURRENT
/ 
-- grant needed permissions
GRANT DBA to c##myadmin                            ;
GRANT CREATE SESSION TO c##myadmin                 ;
GRANT CREATE TABLE TO c##myadmin                   ;
GRANT EXECUTE_CATALOG_ROLE TO c##myadmin           ;
GRANT EXECUTE ON DBMS_LOGMNR TO c##myadmin         ;
GRANT SELECT ON V_$DATABASE TO c##myadmin          ;
GRANT SELECT ON V_$LOGMNR_CONTENTS TO c##myadmin   ;
GRANT SELECT ON V_$ARCHIVED_LOG TO c##myadmin      ;
GRANT SELECT ON V_$LOG TO c##myadmin               ;
GRANT SELECT ON V_$LOGFILE TO c##myadmin           ;
GRANT RESOURCE, CONNECT TO c##myadmin              ;

Then:
sqlplus c##myadmin/myadmin

BEGIN
  DBMS_LOGMNR.ADD_LOGFILE(LogFileName=&gt;'/fra/RYCDB19/archivelog/2021_01_18/o1_mf_1_130_j0bo2onq_.arc',Options=&gt;DBMS_LOGMNR.new);
  DBMS_LOGMNR.START_LOGMNR(Options=&gt;DBMS_LOGMNR.DICT_FROM_ONLINE_CATALOG+DBMS_LOGMNR.NO_ROWID_IN_STMT);
END;
/

PL/SQL procedure successfully completed.

SQL&gt; select count(*) from v$logmnr_contents;

  COUNT(*)
----------
    159587

Whenever you use ALTER SESSION set &quot;_ORACLE_SCRIPT&quot;=true to create your own users and objects, they have their ORACLE_MAINTAINED property set to Y. This is not supported and can break functionality of features.