Error reloading settings

I saved my configuration on my net 900 central router but when I try to reload it onto router I get the following message;

Invalid image file Please select the correct image file and upload it again.

Any suggestions?

The file you saved should be config.bin unless you named it something else. Are you trying to restore on the same screen as you would do a firmware update? The config.bin should be restored using the same screen (system) that you used to save the file.

I have done exactly that. Used config.bin from system menu to restore settings.
For some reason I am getting the error.