Started getting this error yesterday/today. No issues before then. So far, seems to be associated with just one MCC. Can provide more details if needed to AdWords API team (I have Fiddler capture of exchange) and is readily reproducible today.
<?xml version="1.0" encoding="utf-8"?>
<soap:Header>
</RequestHeader>
</soap:Header>
<soap:Body>
<serviceSelector>
<fields>AdServingOptimizationStatus</fields>
<fields>AdvertisingChannelType</fields>
<fields>Amount</fields>
<fields>BidCeiling</fields>
<fields>BidType</fields>
<fields>BiddingStrategyId</fields>
<fields>BiddingStrategyName</fields>
<fields>BiddingStrategyType</fields>
<fields>BudgetId</fields>
<fields>BudgetName</fields>
<fields>BudgetReferenceCount</fields>
<fields>BudgetStatus</fields>
<fields>DeliveryMethod</fields>
<fields>Eligible</fields>
<fields>EndDate</fields>
<fields>EnhancedCpcEnabled</fields>
<fields>FrequencyCapMaxImpressions</fields>
<fields>Id</fields>
<fields>IsBudgetExplicitlyShared</fields>
<fields>Level</fields>
<fields>Name</fields>
<fields>Period</fields>
<fields>PricingMode</fields>
<fields>RejectionReasons</fields>
<fields>ServingStatus</fields>
<fields>Settings</fields>
<fields>StartDate</fields>
<fields>Status</fields>
<fields>TargetContentNetwork</fields>
<fields>TargetGoogleSearch</fields>
<fields>TargetPartnerSearchNetwork</fields>
<fields>TargetSearchNetwork</fields>
<fields>TimeUnit</fields>
<paging>
<startIndex>0</startIndex>
<numberResults>50</numberResults>
</paging>
</serviceSelector>
</get>
</soap:Body>
</soap:Envelope>
<?xml version="1.0" encoding="utf-8"?>
<soap:Body>
<soap:Fault>
<faultcode>soap:Server</faultcode>
<faultstring>InternalApiError.UNEXPECTED_INTERNAL_API_ERROR @ com.google.ads.api.services.common.error.InternalApiError.<init>(InternalApiErro</faultstring>
<detail>
<message>InternalApiError.UNEXPECTED_INTERNAL_API_ERROR @ com.google.ads.api.services.common.error.InternalApiError.<init>(InternalApiErro</message>
<ApplicationException.Type>ApiException</ApplicationException.Type>
<fieldPath></fieldPath>
<trigger></trigger>
<errorString>InternalApiError.UNEXPECTED_INTERNAL_API_ERROR</errorString>
<ApiError.Type>InternalApiError</ApiError.Type>
<reason>UNEXPECTED_INTERNAL_API_ERROR</reason>
</errors>
</ApiExceptionFault>
</detail>
</soap:Fault>
</soap:Body>
</soap:Envelope>