Hi @timowevel
That makes sense since you have both Mattermost and Jitsi installed in the same server and both are trying to use port 443 (HTTPS) at the same time.
Would you consider installing any of the app (either Jitsi / Mattermost) on a different server or consider using other ports for secure connection - https://stackoverflow.com/questions/29414208/can-i-use-another-port-other-than-443-for-https-ssl-communication ?