@mxsh your colleague Johan already has an open ticket with us to discuss and diagnose this issue but has not yet responded to our helpdesk reply (ref 2340).
You can confirm your server has connectivity to the Let's Encrypt API using PowerShell:
Invoke-WebRequest -Uri https://acme-v02.api.letsencrypt.org/directory which should return a StatusCode 200 (OK). Note that you must have TLS1.2 enabled to communicate with the Let's Encrypt API, I suggest using the IISCrypto tool by Nartec to configure their Best Practises mode to enable general purpose communication.
You are also not using the latest version of the app [which is 5.6.8].
Note that you can enable debug http logging by editing C:\ProgramData\certify\serviceconfig.json and set the "LogLevel" field to "debug" instead of "information", then restart the Certify background service and attempt your request again (just click "Request Certificate" on a managed certificate. The C:\ProgramData\certify\logs\session*.log file will then include the actual http conversation between you and the CA (which we are assuming is Let's Encrypt). [You can send this log file through to support at certifytheweb.com rather than posting anything here]