findMeetingTimes throws 500 error, code unchanged - microsoft-graph-api

Question

Our code started throwing the following error today, for a very simple findMeetingTimes request.
This is the error:
{
"error": {
"code": "UnknownError",
"message": "{\"ClassName\":\"System.ArgumentNullException\",\"Message\":\"Value cannot be null.\",\"Data\":null,\"InnerException\":null,\"HelpURL\":null,\"StackTraceString\":\" at System.String.Join(String separator, String[] value)\\r\\n at Microsoft.Griffin.WebService.Calendar.Scheduling.DefaultController.<ExecuteWorkflow>d__53`2.MoveNext()\",\"RemoteStackTraceString\":null,\"RemoteStackIndex\":0,\"ExceptionMethod\":\"8\\nJoin\\nmscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089\\nSystem.String\\nSystem.String Join(System.String, System.String[])\",\"HResult\":-2147467261,\"Source\":\"mscorlib\",\"WatsonBuckets\":null,\"ParamName\":\"value\"}",
"innerError": {
"request-id": "f9153038-b8e2-41a8-bba6-dad2a8cc3afa",
"date": "2019-09-10T12:01:37"
}
}
This is the POST request
{
"attendees": [{
"emailAddress": {
"address": "amy#domain.com"
},
"type": "required"
}],
"timeConstraint": {
"timeslots": [{
"start": {
"dateTime": "2019-09-10T12:01:36",
"timeZone": "UTC"
},
"end": {
"dateTime": "2019-09-17T12:01:36",
"timeZone": "UTC"
}
}]
},
"meetingDuration": "PT30M",
"maxCandidates": 4,
"minimumAttendeePercentage": 100
}
Anyone got an idea what might be causing this?
// Edit:
Some more testing revealed that this is still working with a bearer token from Microsoft Graph Explorer, but not our own token which generated from an AzureAD app registration. From what we understand the tokens should be both valid (and have been up until a few hours ago... Our token also works for other endpoints)
Our token data:
{
"aud": "https://graph.microsoft.com",
"iss": "https://sts.windows.net/0e7219f7-4c4d-47e7-b98a-7b9b9de02b80/",
"iat": 1568152509,
"nbf": 1568152509,
"exp": 1568156409,
"acct": 0,
"acr": "1",
"aio": "******",
"amr": [
"pwd",
"mfa"
],
"app_displayname": "Outlook Meetings for Jira",
"appid": "fb930189-fe2c-4b11-b424-f576f7b371b3",
"appidacr": "1",
"family_name": "*******",
"given_name": "*****",
"ipaddr": "185.85.64.210",
"name": "*****",
"oid": "12ad9a79-78ae-424a-b6ef-a6ec78f69f3f",
"platf": "3",
"puid": "10033FFF8670ACD2",
"scp": "Calendars.ReadWrite.Shared email offline_access User.Read User.ReadBasic.All",
"signin_state": [
"kmsi"
],
"sub": "QsD80H0dBCYK4wttPZFIY1w_Pi5Ygbn9OwKuiNkNPXo",
"tid": "0e7219f7-4c4d-****-b98a-7b9b9de02b80",
"unique_name": "...#....com",
"upn": "...#....com",
"uti": "uzKz01rSV0SD1kTwBZJkAA",
"ver": "1.0",
"wids": [
"62e90394-69f5-4237-9190-012177145e10"
],
"xms_tcdt": 1374053072
}