Hello,
I can’t find the setting so I decided to ask here.
How can I make the MUC (Groupchat) available to everyone. Also to users outside my Server?
I would be very happy if someone could help me 
You have to establish Server to Server connection with that remote server (check http://yourserver:9090/server2server-settings.jsp). Then the users from the other server should be able to browse your MUC service and join chats.
Service Activated - Remote Servers are allowed to connect [5269]
Inactive connections are closed after 10 Minutes.
Permit connection - everyone.
that is my configuratoin
Instant Messages are send correctly to other Servers
In the DNS the whole domain is set to one domain (domain.tld, *.domain.tld).
I have now also set conf.domain.tld to the same IP-Address.
But I think that this is not solving the problem.
About two months ago I had the same problem, but after adding an DNS entry and waiting for the cache updates our server is reachable by other servers.
dig SRV _xmpp-server._tcp.conference.cs.uni-potsdam.de ; <<>> DiG 9.4.3-P2 <<>> SRV _xmpp-server._tcp.conference.cs.uni-potsdam.de
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 18237
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0 ;; QUESTION SECTION:
;_xmpp-server._tcp.conference.cs.uni-potsdam.de. IN SRV ;; ANSWER SECTION:
_xmpp-server._tcp.conference.cs.uni-potsdam.de. 43200 IN SRV 10 10 5269 lisa.cs.uni-potsdam.de.
Best regards
Yeah, that was the answer thanks!