I heard that the buildings were insured just before terrorist attacks. Is that true? If yes, wouldn't this be an argument for conspirationists ?
It's absolutely not true, and conspiracy adherents have no case what so ever.
First of all, in the 1993 bombing, insurers paid out $510 million in that terrorist attack. That was quite a few years before the 9/11/01 event, and would blow a hole in that nut-ball idea.
As if that isn't enough reality to dissuade these "adherents", the Wharton School at the University of Pennsylvania states (PDF)
If anything, terrorism coverage is actually the norm, not an exception.
As others have said, the hub-bub was more about who got paid what by whom. Due to the way insurance policies work, in combination with real estate laws, all the leasing of the property, and switching carriers... It's the sort of stuff that makes a person's head swim.
Sorry for being so derisive in my answer, but the whole 9/11 "truther" conspiracy is one of the most insulting, if not downright stupid, ideas that seems to percolate on the internet. They continually persist no matter what debunking is provided, and only proclaim that all debunking is part of the conspiracy. As someone that served before, during, and after 9/11, and had my life personally affected by that horrible day, it just boggles the mind how disconnected from reality some people can be. They are as deluded as holocaust deniers, moon hoax proponents, or whatever patently ridiculous nuttery people glom on to...
Because the lease on the buildings had only be signed 6 weeks before the attacks, there was a legal dispute which wasn't resolved until 2007. As noted in this NY Times article:
The issue was not that the buildings hadn't been insured, but which policy was in effect: the old one, or the new one which was still being finalized.
So, to answer your question, yes, it seems quite clearly the case that, just six weeks prior to the attacks, the buildings were insured as part of the deal mentioned above.
As to whether it means anything, when coupled with the claim that the WTC towers were laden with asbestos and needed to be demolished and rebuilt, the conspiracy theorists present it as an argument in favour of the conspiracy theory.