Looks like you translated back the error message from a non-English version of VC++. The MSDN page on the error message at http://msdn.microsoft.com/en-us/library/y59181c6.aspx doesn't quite tell me much more than that, but the original message is a bit more informative:
'identifier1' : C linkage function cannot return C++ class 'identifier2'
And though that is not the exact original meaning, in the context here the message probably means in essence that a native function can't return a managed type. This is speculation to some extent, though, since the code you posted doesn't really tell the whole story. Is, by any chance, perhaps server->Receive() a native function or is any other native function involved? Since the error message mentions C linkage, the function in question may not be a member of a class at all (or it may be static), and perhaps it's just unmanaged by mistake.
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