Daikin Onecta integration fails during OAuth: unauthorized_client The client was not found

Third party integrations
1

Hello,

I am trying to integrate two Daikin air conditioning units with Home Assistant using the Daikin Onecta integration.

Home Assistant version:

  • Installation Method: Home Assistant OS
  • Core: 2026.6.3
  • Supervisor: 2026.06.1
  • Operating System: 17.3
  • Frontend: 20260527.5

Air conditioning units:

Model: ATXP25N5V1B9
Serial Number: T003657

Model: ATXP25N5V1B9
Serial Number: T003725

Both units work correctly through the Onecta mobile application.

When I try to add the Daikin Onecta integration, the OAuth login page returns:

Error code: unauthorized_client

Error details: The client was not found.

The integration cannot be completed and no devices are discovered.

Has anyone experienced this issue recently or knows if there is a problem with the Daikin Onecta OAuth configuration?

Thank you very much for any help.