Do not let this complexity scare you. Most of you will be fine with default config file (or light changes).
fehlen Jabber elementare Eigenschaften eines Messengers - z.B. mehrere Clients pro Sitzung,
At the time of original writing of this XEP, many XMPP servers handle message stanzas sent to a user@host (or "bare") JID with no resource by delivering that message only to the resource with the highest priority for the target user. Some server implementations, however, have chosen to send these messages to all of the online resources for the target user. If the target user is online with multiple resources when the original message is sent, a conversation ensues on one of the user's devices; if the user subsequently switches devices, parts of the conversation may end up on the alternate device, causing the user to be confused, misled, or annoyed.
After finally having worked out most of ejabberd's quirks I was playing with some Jabber clients when I finally found out that Jabber/XMPP lacks one of the most essential messenger's features: The ability to use more than one client at once! ... The frequent change of your IP address seems to cause problems in Jabber.