Request returning 403 Forbidden after months of working properly

My integration is a user account that accesses a Confluence page and makes changes according to another custom Slack slash command. For months (years, in fact) it has been working properly but a few weeks ago I found that the requests were not saving to the page properly. Making the requests manually I see that there is a 403 error, with no entries in the errors array.

  "statusCode": 403,
  "data": {
    "authorized": false,
    "valid": true,
    "errors": [
    "successful": false

I also don’t see any changes to the page permissions. How do I see why the page is no longer accessible?

After some digging I found that the Atlassian API has moved to API tokens so I logged in as the original user and generated a token, encoded it base64 and sent it as a basic auth header. I was able to get back to my original functionality, so thumbs up