Hi. Don't think there are more details to provide. The error message appears upon pressing C-c C-c (bound to the org-ctrl-c-ctrl-c command) on a headline, doesn't matter which level. It happened in 3 different Org files, one of which was created for testing. Also tried with "emacs -q" but the error persisted.
Strange message though, org-tags-completion-function is a function, not a variable, so why is Emacs complaining that it isn't a list? Probably related to commit acf7f47ecd20a48c05f97dc660b00d1850b57e10?
