Error getting OAuth access token. cURL error 7: Failed to connect

Hi,

I've set up a gitlab and nextcloud instance using docker-compose (behind a plesk proxy that redirect http to https) on my server recently and I am quite happy with them. However, I struggle a bit with the gitlab integration app in nextcloud.

Basically, I've followed the instructions and created an application on gitlab, called "Nextcloud" and with the following permissions

On nextcloud, I've filled the app ID and secret keys as informed, but when I try to connect my user, I get the following error message:
Error connecting to GitLab: Error getting OAuth access token. cURL error 7: Failed to connect to git.XXXX.com port 443: Connection refused (see https://curl.haxx.se/libcurl/c/libcurl-errors.html) for https://git.XXXX.com/oauth/token

I don't see much more information on the server logs or in the console so I'm a bit confused how to debug this issue.

As a test, I've tried manual curl calls to https://git.XXXX.com/api/v4/user in a terminal (at different locations) but get a 404 error, although I receive an response when I type this address in firefox directly. I can also see the request on the nginx logs from the gitlab docker:
192.168.128.1 - - [25/Aug/2021:22:30:12 +0000] "GET /oauth/authorize?client_id=XXX&redirect_uri=https%3A%2F%2Ffiles.XXXX.com%2Fapps%2Fintegration_gitlab%2Foauth-redirect&response_type=code&state=z160x7sz4&scope=api%20read_user%20read_api%20read_repository HTTP/1.0" 302 221 "" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:91.0) Gecko/20100101 Firefox/91.0" -

Does anyone have an idea? Thank you!

Start with the OAuth token request to https://git.XXXX.com/oauth/token and compare the reported cURL failure with the GitLab Docker and nginx logs. Reproduce the connection from the Nextcloud environment and determine which proxy or networking configuration prevents the request; done means the OAuth connection succeeds or the required deployment fix is documented.