The link you've provided already gives the (probable) answer: you've selected the GTRR option “Match found set”, and while the records of the found set have at least one related record between them, the current record at the moment of script execution doesn't.
Check for error #401 instead, which will be thrown if there is not a single related record for the found set.
and btw, If [ not Get(LastError) = 0 ] is the same as If [ Get ( LastError ) ]
THAT makes perfect sense. Thanks. Really helpful - especially as my next GTRR is the same so could malfunction because of the error 0. I will go and change it.