Kindle PW can't open MOBI output, no prob with PC app

I've been using Calibre for a long time to convert EPUB files to MOBI for use with my Kindle PW (latest model). I'm currently using Calibre 3.2.1, and for some reason recently I can never open its MOBI output on the PW. If I try to email it (or Send to Kindle), it never appears, and if I sideload via USB, it says it can't open the file. But the same file works fine with the Kindle app on Windows 10.

The only way I can use it now is to convert to AZW3 and sideload it via USB, because for some reason AZW3 doesn't work with email or Send to Kindle.

Any suggestions, please?