Following steps will ease up the handling of space problems in the flash recovery area.

1. Determine what is using the space in the flash recovery area

Issue the following statement in the affected database:
select * from v$flash_recovery_area_usage;

2. Check the columns PERCENT_SPACE_USED and PERCENT_SPACE_RECLAIMABLE.
PERCENT_SPACE_RECLAIMABLE shows how much space can be reused by Oracle in case that space is needed for higher prioritized files.
e.g. if there are flashback logs which are older than guaranteed by db_flashback_retention_target then this space is reclaimable as it will be overwritten if space is needed for archived redo logs.

If PERCENT_SPACE_USED is high and PERCENCT_SPACE_RECLAIMABLE is low for ARCHIVELOG ==> Step 3
If PERCENT_SPACE_USED is high and PERCENCT_SPACE_RECLAIMABLE is high for ARCHIVELOG ==> Nothing needs to be done
If PERCENT_SPACE_USED is high and PERCENCT_SPACE_RECLAIMABLE is high for FLASHBACKLOG ==> Nothing needs to be done
If PERCENT_SPACE_USED is high and PERCENCT_SPACE_RECLAIMABLE is low for FLASHBACKLOG ==> Step 4

3. Backup the archived redo log files with RMAN

4. Determine the existence of restore points
Issue the following statement in the affected database:
select name, guarantee_flashback_database, to_char(time, 'DD.MM.YYYY - HH24:MI:SS') from v$restore_point;
If there is a restore point where guarantee_flashback_database = YES ==> Step 5
If there is no restore point where guarantee_flashback_database = YES ==> Step 6

5. Drop the guaranteed restore point
Issue the following statement in the affected database:
drop restore point RESTORE_POINT_NAME;

Validate success with statement in Step 1

6. Validate the setting for db_flashback_retention_target
This parameter defines the guaranteed flashback time in minutes in the database.
If you have a lot of transactions during the day this will eat up your space.

It's better to lose flashback guarantees than facing a database which is stuck
==> reduce the setting of db_flashback_retention_target to an appropriate value with the following statement:
alter system set db_flashback_retention_target = 120; -- to reduce the value to 2h
Validate the success with the statement from Step 1