I'd also recommend resetting the remote. Hold main and stop for 15 seconds. Then click all. Then reload file. Might help

I'd tried that ... after the remote locked up upon download, I pulled the batteries until it died, then replaced them; once the remote finished "booting" (the checkup sequence), I performed a MAIN+STOP reset back to MX defaults, then applied a "blank" file to the remote - the blank file completed download successfully. At this point I downloaded the actual config, which also completed download successfully.

It's upon subsequent attempts that the download continues to fail & remote locks up, now :(