[Sign In/Sign Out - v14.0.0] Problem with Ruby on Sign In and Sign out

When users sign in or sign out, Gitlab goes into an error 500.

In production.log :

On Sign In

Started POST "/users/auth/ldapmain/callback" for 172.18.0.3 at 2021-06-24 10:16:14 +0200
  
ArgumentError (marshal data too short):
  
lib/gitlab/middleware/read_only/controller.rb:50:in `call'
lib/gitlab/middleware/read_only.rb:18:in `call'
lib/gitlab/middleware/same_site_cookies.rb:27:in `call'
lib/gitlab/middleware/handle_malformed_strings.rb:21:in `call'
lib/gitlab/middleware/basic_health_check.rb:25:in `call'
lib/gitlab/middleware/handle_ip_spoof_attack_error.rb:25:in `call'
lib/gitlab/middleware/request_context.rb:21:in `call'
config/initializers/fix_local_cache_middleware.rb:11:in `call'
lib/gitlab/middleware/rack_multipart_tempfile_factory.rb:19:in `call'
lib/gitlab/metrics/requests_rack_middleware.rb:74:in `call'
lib/gitlab/middleware/release_env.rb:12:in `call'

On Sign out

Started POST "/users/sign_out" for 172.18.0.3 at 2021-06-24 10:17:18 +0200
  
ArgumentError (marshal data too short):
  
lib/gitlab/middleware/read_only/controller.rb:50:in `call'
lib/gitlab/middleware/read_only.rb:18:in `call'
lib/gitlab/middleware/same_site_cookies.rb:27:in `call'
lib/gitlab/middleware/handle_malformed_strings.rb:21:in `call'
lib/gitlab/middleware/basic_health_check.rb:25:in `call'
lib/gitlab/middleware/handle_ip_spoof_attack_error.rb:25:in `call'
lib/gitlab/middleware/request_context.rb:21:in `call'
config/initializers/fix_local_cache_middleware.rb:11:in `call'
lib/gitlab/middleware/rack_multipart_tempfile_factory.rb:19:in `call'
lib/gitlab/metrics/requests_rack_middleware.rb:74:in `call'
lib/gitlab/middleware/release_env.rb:12:in `call'

It looks like a problem with Ruby

Start by reproducing the POST requests to /users/auth/ldapmain/callback and /users/sign_out, then inspect lib/gitlab/middleware/read_only/controller.rb:50 and the surrounding middleware stack shown in production.log. Done means sign-in and sign-out no longer return HTTP 500 with ArgumentError (marshal data too short).