ORA-30036: Only Active undo segments and no running transaction

Hello,

the database I am using (Oracle Database 12c - 12.2.0.1.0 - 64bit Production) is having massive issues with the undo tablespace. The undo table space is completely used (aprox. 19 GB not extendable) and every segment is active but there are no transactions running. It is using auto undo management. The undo table space recommendation sql script from the support page didn't raise any errors. Flashback Archive is activated on two schemas.

Issues

Among the massive problems are: no successful login in Oracle SQL Developer with error ORA-30036. Applications using this database cannot perform any actions on the database and in the log you can also find error ORA-30036. Running transactions under 'v$transaction' cannot be found on the database and all segments under 'dba_undo_extents' of the undo table space are neither unexpired nor expired, they all have the status active. No matter how long you wait, the segments will not be released again.