Recently started getting bad response when calling SearchPeople

[andycouzens-egress]

Our code hasn't changed and we're on the latest nuget - the failing code is:

            var view = new EWS.ItemView(10);
            var result = await (await GetExchangeService()).SearchPeople(view, filter);

When this code is invoke an exception is thrown, using Fiddler we can see the generated request and response.

<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:m="http://schemas.microsoft.com/exchange/services/2006/messages" xmlns:t="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
soap:Header
<t:RequestServerVersion Version="Exchange2015" />
</soap:Header>
soap:Body
<m:FindPeople Traversal="Shallow">
<m:ItemShape>
<t:BaseShape>AllProperties</t:BaseShape>
</m:ItemShape>
<m:IndexedPageItemView MaxEntriesReturned="10" Offset="0" BasePoint="Beginning" />
<m:QueryString>andy couzens</m:QueryString>
<m:SearchPeopleSuggestionIndex>true</m:SearchPeopleSuggestionIndex>
<m:Context />
<m:QuerySources>
<t:Source>Auto</t:Source>
</m:QuerySources>
</m:FindPeople>
</soap:Body>
</soap:Envelope>

<s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"><s:Header><h:ServerVersionInfo MajorVersion="15" MinorVersion="20" MajorBuildNumber="4065" MinorBuildNumber="27" Version="V2018_01_08" xmlns:h="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"/></s:Header><s:Body>An internal server error occurred. The operation failed., Auto are unrecognized sources.ErrorInternalServerError0000</s:Body></s:Envelope>

This is when calling O365 - tried using a few different accounts with the same result.