401 Authentication failure Token error

We have implemented backlog API integration with our service.
One of our customer reported that OAuth token gave “message: Authentication failure”,“code:11” token error after 1month or so from the time of issuing OAuth token.
The customer said they have not modified anything on our end.
What would be the possible cause?

Hi there,

Thank you for your message and apologies for the issues occurring. Could you please email us the inquiry and Space details via the contact form below for us to investigate the issue further?