When I do C-c c, I got an error:

org-capture-expand-file: Invalid file location: nil

But (org-capture-expand-file "") returns the expected value. And the file does exist. But (org-capture-expand-file nil) raise an error. So I guess the function was given an erroneous value. 

My relevant settings are:

(setq org-default-notes-file (concat org-directory "/.notes"))

I did not change my configurations for a long time. Maybe some of them are outdated?

Thanks,

David