I had a similar situation; in my case I had been having some issues with Flashback Data Archive (Also Known as FBA, FBDA, FDA) with some queries, possibly due to materialized views. So to solve that problem, I dis-associated those tables from the FBDA, using DBMS_FLASHBACK_ARCHIVE.DISASSOCIATE_FBA(owner_name, table_name)
which solved the performance problem, but of course those tables were no longer tracked.
Now, I've given up on the FBA on that instance for now, and when cleaning up, I hit the same ORA-55641 and also ORA-55626 when trying to clean up.
What I ended up doing was to re-associate the tables to the flashback data archive, and also (not sure if this was needed, but ...) I purged all records:
alter flashback archive ARCH_FLASHBACK_10_YEAR purge all;
-- Identify tables tied to the flashback archive; mine was named ARCH_FLASHBACK_10_YEAR
select owner_name, table_name, FLASHBACK_ARCHIVE_NAME,
ARCHIVE_TABLE_NAME, status from DBA_FLASHBACK_ARCHIVE_TABLES;
-- look for ones with a STATUS of "DISASSOCIATED" and do the next two
-- statements for those tables
exec DBMS_FLASHBACK_ARCHIVE.REASSOCIATE_FBA ('YOUR_OWNER', 'YOUR_TABLE')
alter table YOUR_OWNER.YOUR_TABLE no flashback archive;
-- Then query again. When clean:
alter flashback archive ARCH_FLASHBACK_10_YEAR remove tablespace ARCH_HIST;
drop flashback archive ARCH_FLASHBACK_10_YEAR;
-- And if ARCH_HIST has no other data:
drop tablespace ARCH_HIST including contents and datafiles;
And after doing those steps I was able to drop the flashback archive, and drop the tablespace associated with that flashback archive.