I am in the process of updating my 5.2 rails to 7, now at step 6.0 to 6.1 get a HPE_Header_OVERFLOW when starting app

when I call my app via the link in the browser it comes with 'HPE_HEADER_OVERFLOW: Request could not be proxied'

the rails trace looks clean:
'Started GET "/d/users/sign_in" for 73.127.89.253 at 2022-03-23 21:52:51 +0000
Cannot render console from 73.127.89.253! Allowed networks: 127.0.0.0/127.255.255.255, ::1
Processing by Devise::SessionsController#new as HTML
Rendering layout layouts/application.html.erb
Rendering devise/sessions/new.html.erb within layouts/application
Rendered devise/shared/_links.html.erb (Duration: 1.3ms | Allocations: 526)
Rendered devise/sessions/new.html.erb within layouts/application (Duration: 470.4ms | Allocations: 412889)
Rendered layouts/_flash_messages_dro.html.erb (Duration: 0.3ms | Allocations: 154)
Rendered layouts/_header.html.erb (Duration: 3.3ms | Allocations: 2347)
Rendered layout layouts/application.html.erb (Duration: 700.8ms | Allocations: 652111)
Completed 200 OK in 736ms (Views: 705.6ms | ActiveRecord: 4.8ms | Allocations: 672243)'

For background: I am developing on AWS-Cloud9. The app ran great in 5.1, and also great on Rails6.0. Then I updated ruby to 3.1.1 and then rails to 6.1 and worked out the errors that would show in the logs. Now when I start the server as follows: with a run script: '
export PATH=/home/ubuntu/.rvm/gems/ruby-3.1.1/bin:/home/ubuntu/.rvm/gems/ruby-3.1.1@global/bin:/home/ubuntu/.rvm/rubies/ruby-3.1.1/bin:/home/ubuntu/.rvm/bin:/home/ubuntu/.nvm/versions/node/v6.17.0/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/home/ubuntu/.local/bin:/home/ubuntu/bin:/snap/bin

export GEM_HOME=/home/ubuntu/.rvm/gems/ruby-3.1.1
export GEM_PATH=/home/ubuntu/.rvm/gems/ruby-3.1.1:/home/ubuntu/.rvm/gems/ruby-3.1.1@global

cd /home/ubuntu/environment/twf

rails server -u puma -b localhost'

the server starts just fine, and when I call the link it does seem to start per rails log, and completes 200 OK but now I do get a browser message: 'HPE_HEADER_OVERFLOW: Request could not be proxied!
There was an error proxying the request.'

Does anybody have a hint for me on where to start looking. I googled it and found a page in Japanese from 2017 but that was for Phyton on cloud9 and not rails.

Any help is greatly appreciated.... even questions I would have to answer to get more background are all good.

not asking for an absolute solution, but just a hint to know in what direction to look, or what to exclude to see if it works