I finally found a temporary work around:
I disabled fork by setting $useFork to false in updatesearchindexsolr.php.
I can't figure out what exactly happens but I think it is in relation with concurrent processes running. I noticed that indexer stops when it finishes indexing the first top node...
I use PHP 5.2.0.
I tried with both UsePersistentConnection=enabled and UsePersistentConnection=disabled, no success. Is there a more recent version of eZ Find for eZ Publish 4.0 available over there?