OK, I have made some progress, no the last block of code is:

spb:=spb+char(isc_spb_version)+char(isc_spb_current_version);
spb:=spb+char(isc_action_svc_properties);
spb:=spb+char(isc_spb_dbname)+char(Length(dbname))+char(Length
(dbname) shr 8)+dbname;
spb:=char(isc_spb_options); //THIS SHOULD BE USED!!! - this line
removed 'unrecognized...' error
spb:=spb+char(isc_spb_prp_db_online);
stat:=ISCServiceStart(@sv,@serviceHandle,nil,Length(spb),pchar(spb));
lblStartStatusCode.Caption:=IntToStr(stat);

But now I have error 335544792 - 'Cannot attach to services manager',
although, e.g. @serviceHandle is not null, this is even more strange,
because the function isc_attach_service already was executed without
any errors...