Hello Rodolfo, the error occurs on create subnet, I will include an excerpt here and I have asked the reporter to publish what he can:

2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers [req-9dd64374-b23d-4296-b0f4-98b3cab64490 8a40f8aceae640a2ae10510992073b05 645ffd60a1054fab8359e3afe7244572 - 7a9fab9568114905b8c864caaa6468e1 7a9fab9568114905b8c864caaa6468e1] Mechanism driver 'ovn' failed in create_subnet_postcommit: neutron_lib.exceptions.SubnetInUse: Unable to complete operation on subnet 34fd86b6-5030-4e58-b559-964088afdc1d: This subnet is being modified by another concurrent operation.
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers Traceback (most recent call last):
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron/plugins/ml2/managers.py", line 477, in _call_on_drivers
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     getattr(driver.obj, method_name)(context)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron/plugins/ml2/drivers/ovn/mech_driver/mech_driver.py", line 428, in create_subnet_postcommit
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     context.network.current)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovn_client.py", line 2017, in create_subnet
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     self.update_metadata_port(context, network['id'])
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/ovn_client.py", line 2206, in update_metadata_port
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     metadata_port['id'], port)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron/common/utils.py", line 687, in inner
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     return f(self, context, *args, **kwargs)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron_lib/db/api.py", line 233, in wrapped
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     return method(*args, **kwargs)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron_lib/db/api.py", line 139, in wrapped
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     setattr(e, '_RETRY_EXCEEDED', True)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/oslo_utils/excutils.py", line 220, in __exit__
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     self.force_reraise()
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/oslo_utils/excutils.py", line 196, in force_reraise
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     six.reraise(self.type_, self.value, self.tb)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/six.py", line 693, in reraise
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     raise value
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron_lib/db/api.py", line 135, in wrapped
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     return f(*args, **kwargs)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/oslo_db/api.py", line 154, in wrapper
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     ectxt.value = e.inner_exc
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/oslo_utils/excutils.py", line 220, in __exit__
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     self.force_reraise()
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/oslo_utils/excutils.py", line 196, in force_reraise
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     six.reraise(self.type_, self.value, self.tb)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/six.py", line 693, in reraise
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     raise value
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/oslo_db/api.py", line 142, in wrapper
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     return f(*args, **kwargs)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron_lib/db/api.py", line 183, in wrapped
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     LOG.debug("Retry wrapper got retriable exception: %s", e)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/oslo_utils/excutils.py", line 220, in __exit__
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     self.force_reraise()
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/oslo_utils/excutils.py", line 196, in force_reraise
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     six.reraise(self.type_, self.value, self.tb)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/six.py", line 693, in reraise
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     raise value
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron_lib/db/api.py", line 179, in wrapped
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     return f(*dup_args, **dup_kwargs)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron/plugins/ml2/plugin.py", line 1656, in update_port
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     port)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron_lib/db/api.py", line 233, in wrapped
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     return method(*args, **kwargs)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron/db/db_base_plugin_v2.py", line 1488, in update_port
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     new_port=new_port)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron/db/ipam_backend_mixin.py", line 709, in update_port
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     new_port.get('mac_address'))
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron/db/ipam_pluggable_backend.py", line 428, in update_port_with_ips
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     new_mac)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron/db/ipam_pluggable_backend.py", line 351, in _update_ips_for_port
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     fixed_ips=changes.add + changes.original)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron/db/ipam_backend_mixin.py", line 664, in _ipam_get_subnets
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     id=subnet.id)
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers   File "/usr/lib/python3/dist-packages/neutron/db/models_v2.py", line 55, in lock_register
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers     raise exception
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers neutron_lib.exceptions.SubnetInUse: Unable to complete operation on subnet 34fd86b6-5030-4e58-b559-964088afdc1d: This subnet is being modified by another concurrent operation.
2020-08-04 14:23:47.914 1220597 ERROR neutron.plugins.ml2.managers 
2020-08-04 14:23:47.915 1220597 ERROR neutron.plugins.ml2.plugin [req-9dd64374-b23d-4296-b0f4-98b3cab64490 8a40f8aceae640a2ae10510992073b05 645ffd60a1054fab8359e3afe7244572 - 7a9fab9568114905b8c864caaa6468e1 7a9fab9568114905b8c864caaa6468e1] mechanism_manager.create_subnet_postcommit failed, deleting subnet '2c8ec745-ca00-41a6-b55f-c9dbfd143868': neutron.plugins.ml2.common.exceptions.MechanismDriverError