Gnus: How to fix 'Warning - invalid active <folder> <number> <number> y'

Question

Something I've done but not yet identified has hosed my working-for-years Gnus<->offlineimap<->GMail setup. Gnus gives me a bunch of errors like:

Warning - invalid active: [Gmail].Starred 26 2 y

for each of the 8 standard GMail folders being synced (Sent, Important, Trash, Spam, All Mail, Bin, Starred, and Drafts) and presents a *Group* buffer consisting of nothing but:

4: nndraft:drafts

instead of the dozens of subscribed groups (mail folders) I expect to see.

I find no relevant matches for 'invalid' in the Gnus manual, nor anything in multiple web searches.

One piece of context: after this first happened, I checked my offlineimap logs and found there were a lot of folder UID errors, so I completely rebuilt my offlineimap local repo from scratch, which took almost 48 hours.

Possibly relevant Gnus user options:

'(gnus-article-sort-functions
     (quote
         ((not gnus-article-sort-by-number))))
'(gnus-check-new-newsgroups nil)
'(gnus-extract-address-components
     (quote mail-extract-address-components))
'(gnus-home-directory "~/.emacs.d/gnus/")
'(gnus-ignored-newsgroups "^to\\.\\|^[0-9. ]+\\( \\|$\\)\\|^[\"]\"[#'()]")
'(gnus-init-file "~/.emacs.d/.gnus.el")
'(gnus-large-newsgroup nil)
'(gnus-parameters
     (quote
         ((".*"
              (visible . t))
             ("INBOX"
                 (display . 75))
             ("All Mail"
                 (display . 75)))))
'(gnus-read-active-file nil)
'(gnus-read-newsrc-file nil)
'(gnus-save-killed-list nil)
'(gnus-save-newsrc-file nil)
'(gnus-select-method
     (quote
         (nnmaildir "GMail"
             (directory "~/GMail/")
             (directory-files nnheader-directory-files-safe)
             (get-new-mail nil))))
'(gnus-started-hook
     (quote
         (ph/gnus-redisplay-groups ph/setup-bbdb)))
'(gnus-startup-hook
     (quote
         (bbdb-insinuate-gnus)))
'(gnus-subscribe-newsgroup-method
     (quote gnus-subscribe-alphabetically))
'(gnus-thread-sort-functions
     (quote
         ((not gnus-thread-sort-by-number)
             (not gnus-thread-sort-by-date))))
'(gnus-use-scoring nil)

gnus-version is "Gnus v5.13" and emacs-version is "24.5.1"