I had the following in my Gemfile...
gem 'omniauth-oauth2', '~> 1.7'

After a bundle update omniauth-oauth2 went from v.1.7.3 -> 1.8.0

That caused errors when authenticating with Slack as follows...
D, [2022-09-07T18:08:29.046543 #42987] DEBUG -- omniauth: (slack_chiirp) Callback phase initiated.
OAuth2::Client#get_token argument extract_access_token will be removed in oauth2 v3. Refactor to use access_token_class on #initialize.
D, [2022-09-07T18:08:29.047586 #42987] DEBUG -- omniauth: (slack) API request '[:post, "https://slack.com/api/oauth.v2.access"]'.
E, [2022-09-07T18:08:29.200274 #42987] ERROR -- omniauth: (slack_chiirp) Authentication failure! undefined method expired?' for nil:NilClass self.access_token = access_token.refresh! if access_token.expired? ^^^^^^^^^: NoMethodError, undefined method expired?' for nil:NilClass
self.access_token = access_token.refresh! if access_token.expired?

Let me know if any other info is needed.