ORA-03826
while Barrier DDL statement with ID string was in progress the number of followers for replication unit string changed.
Cause
While a Barrier DDL statement was being executed across the leader and follower shards of a replication unit, the number of followers changed. This could happen if another leader was shut down, or the current shard transitioned from leader to follower for this replication unit. The current DDL statement was rolled back across all shards in the replication unit.
Action
Execute the DDL statement again when the replication unit and shards stabilize.