Error getting access token for service account: 401 Unauthorized while calling refresh on Google Credentials

I am using a service account to access the google cloud features like Drive and Contact to save data through my app.
I have suddenly started getting this for all users of some clients. Rest of them seem to be working fine.

Error getting access token for service account: 401 Unauthorized
POST https://accounts.google.com/o/oauth2/token, iss: XXXX.iam.gserviceaccount.com

Caused by: com.google.api.client.http.HttpResponseException: 401 Unauthorized
POST https://accounts.google.com/o/oauth2/token
    ... suppressed 3 lines
    ... 61 more

Was and still working fine for certain clients, failing for all users of the other client.
What setting should I verify? Domain wide delegation is correct. Clients say they haven’t changed any setting on their end.

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