I have a miniRouter that has a bad ram chip on it so I saved the config to a file and now I am trying to load said config file on a new miniRouter and it fails to load with error "incorrect file size".

The old board was 2.9.39 and the new board is 2.9.41, do I need to down grade or is there som esimple thing I am missing?

seems your file is either damaged, or something else is wrong with it. this is why you should always make multiple backups, and also make 'export' files, which are human readable and harder to corrupt