Resolved!
Seems the problem related to bad inclusion.
I checked almost 50 methods (really!) to "include" the trackback class and finally made it.
For others facing that problem:
If you write a plugin and try to include a file, do it this way:
include $this->config["base_path"]."path/to/file/include_me.php";Nothing else seems to work fine (beside from writing the whole code of the inclusion file down

)
By the way: TrackBack is working very well.