I have recently started having this issue with my capture templates. In my case I want to ask the user (me) which file should be used to store the note, so I have a bunch of templates like this
(setq org-capture-templates
'(("o" "Project todo" entry (file+headline (av/choose-agenda-file) "Tasks") ))
where av/choose-agenda-file uses completing-read to ask the user to choose a file from a list it constructs on the fly. This approach has worked for years and now it results in the "invalid file location" message. The backquote does not help because the function needs to be called at run time, not when the variable is defined.
Thanks,
Alex