Microsoft Graph SendMail returns ErrorInternalServerError - vb.net

Question

I'm writing a program to send internal emails in VB.Net using the official Microsoft Graph package. At first I got it to send calendar invites and that worked fine for months, but when I tried to add the ability to send emails I keep getting an Internal Server Error.
From looking at other times people have got this error from Graph, it appears to be when a malformed request is sent or when values that don't quite make sense are given. I'm using the constructors from the package and using basically the same construction method as every example online, so the structure of the request can't be wrong, and the only fields that I'm using just take text and don't do anything with it, so I don't think that can be the issue either.
I have the correct permissions in Azure: Screenshot of permissions in Azure, including Mail.Send
Here's my code for constructing the email:
Function DefineImportantEmail(Locations As String, Recipients As List(Of (String, String))) As Message
Dim title = "IMPORTANT OUTAGE NOTIFICATION"
Dim message = "Outage at " & Locations
Dim RecipientList = New List(Of Recipient)
For Each recipient In Recipients
RecipientList.Add(New Attendee With {
.EmailAddress = New EmailAddress With {
.Address = recipient.Item1,
.Name = recipient.Item2
},
.Type = AttendeeType.Optional
})
Next
Return New Message With {
.Subject = title,
.Body = New ItemBody With {
.ContentType = BodyType.Text,
.Content = message
},
.ToRecipients = RecipientList
}
End Function
And here's the code I use to send the email:
Sub SendImportantEmail(Locations As String, Recipients As List(Of (String, String)))
Dim SaveToSentItems = True
Dim graphClient = GetGraphClient()
Dim EventObject = DefineImportantEmail(Locations, Recipients)
Dim request = graphClient.Users("xxxxx#xxxxxxxxxxxxxxx.com").SendMail(EventObject, SaveToSentItems).Request().PostAsync()
request.Wait()
End Sub
The GetGraphClient function works for any other API endpoint I've tried, so I haven't included it. The censored user is from our organisation and is also the user I sent calendar invites from in the first version of this program.
Here's the full error message I get:
(Code: ErrorInternalServerError
Message: An internal server error occurred. The operation failed.
ClientRequestId: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
)
---> Status Code: InternalServerError
Microsoft.Graph.ServiceException: Code: ErrorInternalServerError
Message: An internal server error occurred. The operation failed.
ClientRequestId: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx