I assume you are using Windows? If so, try to create a new file in `C:\Program Files (x86)\Geany\ssl\certs\ca-bundle.crt` with the contents in https://pastebin.geany.org/eR622/, restart Geany and try again (obviously adjust the Geany installation path if it differs).
It might help to restart Geany after creating the file. The pasted CA bundle only contains one of the Let's Encrypt intermediate certificates which is necessary for a proper TLS connection.
If it works then (it did on my Windows test system), then we will ship this certificate with future releases).