If you want to retrieve information of all client accounts under your MCC account, then I suggest that you use
ManagedCustomerService instead. You may refer to
this code sample for reference.
However, to help us narrow down the root cause of the error, could you reply to us privately (using reply privately to author option) with the SOAP request and response captured when error was encountered?
Also, were you able to reproduce the error even with the latest API version (201702)?
Regards,
Ivan
AdWords API Team