XMPPMUC.join("nick") not successfull

Good afternoon,

I have an openfire Server running on myvirtualserver.mydomain

I have a problem joining a MUC which I have created with a working connection.

This is my source code, as it is suggested from many sides:

XMPPConnection conn = new XMPPConnection(“myvirtualserver.mydomain”);

conn.connect();

conn.login(“name”, “password”);

MultiUserChat muc = new MultiUserChat(conn, “channel@myvirtualserver.mydomain”);

muc.join(“nick”);

If I open a vpn channel to “mydomain” and create a Connection via …

new XMPPConnection(“myvirtualserver”)

everything is okay, but otherwise I receive (Pidgin connects to my server in both cases):

No response from server.:
at org.jivesoftware.smackx.muc.MultiUserChat.join(MultiUserChat.java:468)
at org.jivesoftware.smackx.muc.MultiUserChat.join(MultiUserChat.java:362)

Best regards and thanks in advance,

Christian