Http 403 between container jenkins and Gitlab container

I got 2 docker containers with Jenkins V2.410 and another one with Gitlab 16.0.7 on the same physical server.
I setup on Gitlab a personal access token.
On the Jenkins I configure the Gitlab with the following parameters:

Gitlab host URL: http://server:8300 (my Gitlab runs on the port 8300)
Credential: my gitlab credential created previously.
When I make a Test connection, I got the followinf error:
Client error: HTTP 502 bad gateway.
Which is strange because both (jenkins and gitlab) are on the same server.

I tested also with the IP address
gitlab host URL http://MYSERVERIP:8300
And now I got the error:
Client error: HTTP 403 forbidden.

If someone could provide a tips or a solution to investigate.

Fixed! it was my proxy.
I needed to put the Gitlab server ip address in the No proxy host on the Jenkins server.