I think these are all files needed to understand my problem

my_flask_log.log

 * Serving Flask app "simple_python_activitypub"
 * Environment: production
   WARNING: Do not use the development server in a production environment.
   Use a production WSGI server instead.
 * Debug mode: off
 * Running on http://127.0.0.1:5000/ (Press CTRL+C to quit)
127.0.0.1 - - [29/Jul/2018 10:58:44] "GET /users/waweic HTTP/1.0" 200 -
127.0.0.1 - - [29/Jul/2018 10:58:44] "GET /users/waweic HTTP/1.0" 200 -
127.0.0.1 - - [29/Jul/2018 10:58:44] "GET /.well-known/webfinger?resource=acct:waweic@testing1.ism.rocks HTTP/1.0" 200 -

public.pem

-----BEGIN PUBLIC KEY-----
MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAutW8neaZrIGKGI5IWYGa
3aZt7Of09Op+PArt0Sl95MpUjhA44MiKbda6G6qgE2ehjK1GQ+zxLS03D5uKCCpM
3G2BHQVx9WocwJJkwDoB9EWnyJAE/bBtTbkFbMYXgRGVuEadBivbqERDkUOIEiCO
alvZCIcbDJbwLh3TtGVHndIjqYD1Flf1JyIwxhFsXgVdZNroP90fRn7lHsIxH+zK
+DciIqXlryOjpfVG8d6JUzxBMJ8JbTbdmjgqxOoM5Du7QFFPJQdTelLiAsxlIufU
CTko0eqgOIVv4RiKmDRdv4HcgywoPZ04NP2Y4445/NKlB2sW3iPIwUUmxqaYEnAv
3wIDAQAB
-----END PUBLIC KEY-----

ruby_script.log

401
Public key not found for key https://testing1.ism.rocks/users/waweic#main-key

send.rb

require 'http'
require 'openssl'

document      = File.read('create-hello-world.json')
date          = Time.now.utc.httpdate
keypair       = OpenSSL::PKey::RSA.new(File.read('private.pem'))
signed_string = "(request-target): post /inbox\nhost: mastodon.social\ndate: #{date}"
signature     = Base64.strict_encode64(keypair.sign(OpenSSL::Digest::SHA256.new, signed_string))
header        = 'keyId="https://testing1.ism.rocks/users/waweic#main-key",headers="(request-target) host date",signature="' + signature + '"'

http = HTTP.headers({ 'Host': 'mastodon.social', 'Date': date, 'Signature': header })
response = http.post('https://mastodon.social/inbox', body: document)

print(response.code)
print("\n")
print(response.body)
# print(response.body)

# print(signed_string)
# print("\n\n")
# print(signature)

this_is_what_I_send.http

POST /inbox HTTP/1.1
Host: mastodon.social
Date: Sun, 29 Jul 2018 08:53:30 GMT
Signature: keyId="https://testing1.ism.rocks/users/waweic#main-key",headers="(request-target) host date",signature="tgrZ/YvRzx5BuODSFz/IzvUbDvu6o3/grEAwaBnxUsYNmpYH0FNi2gxglFFlWwerSM3cM3Zv+8/wBnWe2i3/g+mtPti9nwVvRo3nTIbalCEXDIxgDpz9STZ4RGNd+jzgW+8/zR+qMgAcVcBU+nvSIXLQNTQdA/fYXrJs9ZI3LkupByzpkfiE7CjSmgfKLmsi4z8EBNJda6EnvnqDLmOroMmgTAI/5LmXfqj1S92E2i3N+bTsXx0YpMyodNMmZXzhQUfJ9zq7G0irZoYPbqBNWqUfZiUau5pSFpS/UBh1J0iurEK7/gYilkOraEewFA6BMFYGuAeA+8bWgfTgSSPMnA=="
Connection: close
User-Agent: http.rb/3.3.0
Content-Length: 582

{
  "@context": "https://www.w3.org/ns/activitystreams",
  "id": "https://testing1.ism.rocks/object/dsngoisgoihs/activity",
  "type": "Create",
  "actor": "https://testing1.ism.rocks/users/waweic",
  "object": {
    "id": "https://testing1.ism.rocks/object/dsngoisgoihs",
    "type": "Note",
    "published": "2018-07-27T20:38:11Z",
    "attributedTo": "https://testing1.ism.rocks/users/waweic",
    "inReplyTo": "https://mastodon.social/users/Gargron/statuses/100254678717223630",
    "content": "<p>Hello world</p>",
    "to": "https://www.w3.org/ns/activitystreams#Public"
  }
}

waweic.json

{"@context": ["https://www.w3.org/ns/activitystreams", "https://w3id.org/security/v1"], "id": "https://testing1.ism.rocks/users/waweic", "type": "Person", "inbox": "https://testing1.ism.rocks/users/waweic/inbox", "preferredUsername": "waweic", "publicKey": {"owner": "https://testing1.ism.rocks/users/waweic", "id": "https://testing1.ism.rocks/users/waweic#main-key", "publicKeyPem": "-----BEGIN PUBLIC KEY-----\nMIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAutW8neaZrIGKGI5IWYGa\n3aZt7Of09Op+PArt0Sl95MpUjhA44MiKbda6G6qgE2ehjK1GQ+zxLS03D5uKCCpM\n3G2BHQVx9WocwJJkwDoB9EWnyJAE/bBtTbkFbMYXgRGVuEadBivbqERDkUOIEiCO\nalvZCIcbDJbwLh3TtGVHndIjqYD1Flf1JyIwxhFsXgVdZNroP90fRn7lHsIxH+zK\n+DciIqXlryOjpfVG8d6JUzxBMJ8JbTbdmjgqxOoM5Du7QFFPJQdTelLiAsxlIufU\nCTko0eqgOIVv4RiKmDRdv4HcgywoPZ04NP2Y4445/NKlB2sW3iPIwUUmxqaYEnAv\n3wIDAQAB\n-----END PUBLIC KEY-----"}}

webfinger.json

{
    "subject": "acct:waweic@testing1.ism.rocks",
    "links": [
        {
            "rel": "self",
            "type": "application/activity+json",
            "href": "https://testing1.ism.rocks/users/waweic"
        }
    ]
}

• I execute the send.rb script.
• The contents like in this_is_what_i_send.http are sent to mastodon.social
• I see the entries in my flask log (at my_flask_log.log). Note that I have an nginx proxy between the internet and my server to provide an SSL connection.
• Both the contents of waweic.json and later webfinger.json are replied to mastodon.social. (Note that I got them over curl, so this definitely works)
• I see the contents of ruby_script.log on my screen and have no clue why I get an error code back, because verifying the signature using public.pem works.

Note, that I nearly completely copy-pasted the contents of send.rb from Gargrons blogpost.