SSO with Letsencrypt certificates on two machines: Bad response from token request!

I have Omnibus Gitlab and Mattermost installed on two separate machines. Both use HTTPS with Let's encrypt certificates and HTTPS works fine.

The problem is that when I use SSO with Gitlab, Mattermost returns the dreadful error: Bad response from token request. The log file is the following (note how the details section is empty!):

[EROR] /signup/gitlab/complete:AuthorizeOAuthUser code=500 rid=o9nzr1yjbtr15xqi5hy8pwssnh uid= ip=68.194.84.101 Bad response from token request [details: ]

I have tried including Let's encrypt certificate authority as explained in other posts, etc. but it did not help. Any Suggestions? Would love to solve this issue.

Thanks!