Should not you be suppling the value to th fetch query string for accountid and/or name? Like building the final parameter as XML string. The error seems to indicate that CRM server side does not like the request parameter. The ULS log may reveal more about the parameter being missing or something.
<fetch mapping='logical'>
<entity name='account'>
<attribute name='accountid'>123</attribute>
<attribute name='name'>SomeName</attribute>
</entity>
</fetch>