confirmed on Windows 
and it is not releated to chaset tested with WIN1250 and NONE - same error 


Message: isc_dsql_fetch failed. 

SQL Message : -902 
Unsuccessful execution caused by a system error that precludes successful execution of subsequent statements 

Engine Code : 335544333 
Engine Message : 
internal Firebird consistency check (invalid SEND request (167), file: JrdStatement.cpp line: 327) 

------------------------------------------------------------------------------------------------------------------------------------------------------ 
in the source code it is 

jrd_req* JrdStatement::findRequest(thread_db* tdbb) 
{ 
    SET_TDBB(tdbb); 
    Attachment* const attachment = tdbb->getAttachment(); 
    const JrdStatement* const thisPointer = this;	// avoid warning 
    if (!thisPointer) 
        BUGCHECK(167);	/* msg 167 invalid SEND request */