Retrieving a PDF document in a SOAP message errors, Base64 sequence length

When I do the call that returns a message which should include a Base64 byte array, I get these messages:

{"Error in deserializing body of reply message for operation 'getObservation'."}
{"There is an error in XML document (1, 878)."}
{System.Collections.ListDictionaryInternal}
{"Base64 sequence length (2) not valid. Must be a multiple of 4."}

Could somebody shed some more light on these messages? What can I do?