Capx are .zip files.
So note the name of the missing file, and go check inside the capx if you find said file.
If you don't, open the main .caproj file (in a text editor) and look for the name of the missing file; remove it.
Do those manipulations on a backup capx, better safe than sorry.
If it still doesn't work, post your capx (unmodified) I'll have a look at it.