UnknownError (503) when calling Graph API's /calendar/events for some users

When using the /calendar/events API, it returns "Unknown Error" for some users, but works OK for others. This has been reproducible consistently for at least 2 months.

Example request:

GET https://graph.microsoft.com/v1.0/users/07f20085-cd3e-4532-a59e-b454de5b62ea/calendar/events?$select=id&$top=1
Authorization: Bearer %token%

And we get the following response (an image encoded as BASE64 omitted for brevity):

  "error": {
    "code": "UnknownError",
    "message": "<html>\r\n<head>\r\n\t<meta charset=\"utf-8\">\r\n\t<meta http-equiv=\"X-UA-Compatible\" content=\"IE=edge\" />\r\n\t<meta name=\"viewport\" content=\"width=device-width, initial-scale=1.0, user-scalable=yes\" />\r\n\t<title>Service Unavailable</title>\r\n</head>\r\n<body>\r\n\t<noindex>\r\n\t\t<table width=\"100%\" height=\"100%\">\r\n\t\t\t<tr>\r\n\t\t\t\t<td align=\"center\" valign=\"center\">\r\n\t\t\t\t\t<img src=\"data:image/png;base64,<..>\r\n\t\t\t\t\t<p style=\"margin-top: 50px;font: 25px sans-serif;\"><span style=\"font-size:33px;font-weight: bold;display: block;margin-bottom:20px;\">Сервис недоступен</span>Service Unavailable</p>\r\n\t\t\t\t</td>\r\n\t\t\t</tr>\r\n\t\t</table>\r\n\t</noindex>\r\n</body>\r\n</html>\r\n",
    "innerError": {
      "date": "2022-01-10T20:24:26",
      "request-id": "ce045d30-8f55-43ee-bddf-25d11f0766a6",
      "client-request-id": "ce045d30-8f55-43ee-bddf-25d11f0766a6"
    }
  }

The error message is unhelpful: there're no details as to what is wrong.

What we did:

1. Made sure the user has a cloud solution (i.e. not on-premises).
2. Asked the organization's admin to check if there's any difference between users for which the API works and for which the API doesn't; apparently, nothing suspect found.

What can be the cause of this "UnknownError" response? Is this a bug on the Teams side, or is it a misconfiguration in the affected organization? Maybe there's some permissions that we're missing (even though it works OK for other orgs)? We're out of ideas.