[Theme: Sunspot] Custom menu disappears from both child and parent themes

I created and saved a custom menu to sit in the left sidebar of my site. For some reason I can’t fathom, it disappears both from the website pages and my menu dashboard, leaving just the menu title with no items. It’s a menu with multiple pages and is a PITA to recreate every time it disappears.

I’ve checked my server logs and today there were lots of “file does not exist” errors, some of which apply to php files. I haven’t tried deactivating my plugins yet.

Any ideas on what’s happening here? Is there any way I can create and save the menu so I have a back up for when this happens again? Thanks in advance.

Site: https://tikatu.com/musemoves/