Connect to router MAC via WinBox (i.e. layer2), upload the file to the RB, then go to terminal and issue
/import verbose=yes file-name=YourExport.rsc
That way you should see where the error is.
Thanks Pukkita, making progress
Found a couple of errors in my config and fixed them but now I got to the point where it just drops the Winbox connection and I cannot get back in. (my config has no config changes to the port I am using to connect to the router and winbox is allowed on all interfaces)
Have not tried rebooting the router yet.
I might try to import my config verbosely from serial console and see what happens.