500 errors when setting up Mattermost integrations

I recently set up Mattermost via the GitLab omnibus config. I have both services working, and both responding over HTTPS with valid certificates (at least as seen in my browser). However, I’m get 505 errors when I try to set up Mattermost notifications and Mattermost slash commands from GitLab. I was able to work around the Mattermost notifications by changing the Mattermost address to the GitLab address for the webhook URL. But I never get an option to enter a URL for Mattermost slash commands. Unlike the screenshot in the documentation, my integration page looks like this:

When I click Add to Mattermost I get another 500 error and can’t get around it. Looking at Sentry the error is OpenSSL::SSL::SSLError: SSL_connect returned=1 errno=0 state=error: certificate verify failed.

Mattermost is installed on the same server as GitLab with a different domain name and different SSL cert. Any suggestions on what to do to get around the 500 errors?