Heya pixelgrid
In the components admin, under "Settings", there is a block called "Addon". Is the checkbox for "Offline Messages" enabled there?
Also in the OpenSim.ini, there must be the section [Messaging] changed to your Joomla url. Something like:
[Messaging]
InstantMessageModule = InstantMessageModule
OfflineMessageModule = OfflineMessageModule
OfflineMessageURL = http://url-to-your-joomla.org/components/com_opensim/interface.php
MuteListModule = MuteListModule
; Change this to your own Joomla url:
MuteListURL = http://url-to-your-joomla.org/components/com_opensim/interface.php
Even if "MuteListURL" is not required, I was told that it needs to be set to the same url like "OfflineMessageURL" to get offline messages working ...
Another reason could be, that in your regions.ini the value of "ExternalHostName" is set to a wrong hostname or IP address.
I hope something helps here
Greetz
FoTo50