Magento integration will not activate

Question

I created a new integration, when I want to activate it, I do get a login pop-up from the external party. I enter my credentials and I got the following message, but the integration is not activated:

Integration '[INTEGRATION NAME]' has been sent for activation.

When I open the integration, the fields "Customer-key" and "Customer-secret are filled in. The fields "Access-token" and "Access-token-secret" are empty.

I got the following log from the external party:

    De oauthdebugclient <br>POST&https%3A%2F%2Fdomain.com%2Foauth%2Ftoken%2Faccess&oauth_consumer_key%3D[CUSTOMER KEY]%26oauth_nonce%3D11455170896229f1fdb48306.85914053%26oauth_signature_method%3DHMAC-SHA1%26oauth_timestamp%3D1646916093%26oauth_token%3D%26oauth_verifier%3Dqf2e6feyal8k9ky11s26i2cj4rnc66vt%26oauth_version%3D1.0<br>POST /oauth/token/access HTTP/1.1
User-Agent: PECL-OAuth/2.0.7
Host: domain.com
Accept: */*
Authorization: OAuth oauth_verifier="qf2e6feyal8k9ky11s26i2cj4rnc66vt",oauth_consumer_key="[CUSTOMER KEY]",oauth_signature_method="HMAC-SHA1",oauth_nonce="11455170896229f1fdb48306.85914053",oauth_timestamp="1646916093",oauth_version="1.0",oauth_token="",oauth_signature="ThrGdBMPNOg%2FySyrXaasqXCbgBA%3D"<br>HTTP/1.1 400 Bad Request
Date: Thu, 10 Mar 2022 12:41:33 GMT
Transfer-Encoding: chunked
Connection: keep-alive
X-XSS-Protection: 1; mode=block
X-Content-Type-Options: nosniff
CF-Cache-Status: DYNAMIC
Expect-CT: max-age=604800, report-uri="https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct" (https://report-uri.cloudflare.com/cdn-cgi/beacon/expect-ct)
Report-To: {"endpoints":[{"url":"https:\/\/a.nel.cloudflare.com\/report\/v3?s=go0%2B%2Fg6SNEA%2BiCSfITo%2FJv1%2FWPiJ3MtoG17KWzl4n5NYra%2F36ZeGgaPyOZpkM6R%2Bjn5vupBvbokGZ2eCX1q5RgTjObJHd%2F%2Fg1GWk7nayMy%2ByPgyv2Rtci%2BfIU%2Be3%2B1MXU2eTotmMq%2B8%3D" (https:\/\/a.nel.cloudflare.com\/report\/v3?s=go0%2B%2Fg6SNEA%2BiCSfITo%2FJv1%2FWPiJ3MtoG17KWzl4n5NYra%2F36ZeGgaPyOZpkM6R%2Bjn5vupBvbokGZ2eCX1q5RgTjObJHd%2F%2Fg1GWk7nayMy%2ByPgyv2Rtci%2BfIU%2Be3%2B1MXU2eTotmMq%2B8%3D)}],"group":"cf-nel","max_age":604800}
NEL: {"success_fraction":0,"report_to":"cf-nel","max_age":604800}
Strict-Transport-Security: max-age=15552000; includeSubDomains; preload
Server: cloudflare
CF-RAY: 6e9c20120ba700da-AMS
alt-svc: h3=":443"; ma=86400, h3-29=":443"; ma=86400<br>0

<br>About to connect() to domain.com port 443 (#8)
  Trying [IP ADDRESS]...
Connected to domain.com ([IP ADDRESS]) port 443 (#8)
  CAfile: /etc/pki/tls/certs/ca-bundle.crt
  CApath: none
SSL connection using TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
Server certificate:
 subject: CN=sni.cloudflaressl.com,O="Cloudflare, Inc.",L=San Francisco,ST=California,C=US
 start date: Jun 11 00:00:00 2021 GMT
 expire date: Jun 10 23:59:59 2022 GMT
 common name: sni.cloudflaressl.com
 issuer: CN=Cloudflare Inc ECC CA-3,O="Cloudflare, Inc.",C=US
Connection #8 to host domains.com left intact

I'm running Magento 2.4.3-p1

Anybody any idea what is going wrong?