Hi cwolff,
Thanks for trying that.
MMM doesn’t do anything when you hit the “Save menu” button (partly because it doesn’t need to, but mostly so we don’t interfere with the complex menu saving process – we leave core WordPress to handle that process on it’s own).
There doesn’t seem to be a definitive answer to this issue, but there are some suggestions/things to try on these threads:
https://www.ads-software.com/support/topic/404403-error-when-saving-menu/
https://www.ads-software.com/support/topic/i-get-a-404-error-when-i-save-my-menu/
https://www.ads-software.com/support/topic/saving-menu-gives-error-404-page-simple-catch/
I seem to remember once the issue was mod_security related, so it may be worth asking your host to investigate.
If you do find a solution please post it back here and it may help someone with the same issue in the future (I’d also be interested to know what caused it!).
Regards,
Tom