5E Sheet - weird red box

Hello, I've been noticing lately that a bunch of my character sheets are having a problem. The bottom entry of my inventory items has a weird red box. I can delete the item, but it comes back, seemingly at random. How do I prevent or fix this?

In my experience that happens when a repeating row is not created correctly . Such as when a new rowID is generated but forced to lower case before creating new repeating row elements /values, or when removing repeating rows the old key is used to create new elements /values, etc. TL;DR, it's a bug in a sheetworker script. Probably one that is creating that repeating row. 

How do I fix it, short of recreating the entire sheet?

This problem goes back at least six years (or at least the symptoms do—the cause might be something else entirely). I.e. It does indeed seem to be a symptom of a malformed row, but the cause of the malformation is likely different in different cases. Is it always the last row? Try deleting the problem row and the row directly above it?