A321 Weight and Balance
 

I had a chance to discuss with a flight dispatcher and I was told that the A321 has very little tolerance for any weight inbalance even at 500 kgr. Is this true and is its legth the reason for this issue. If that is truth, has the B757 the same weakness...

Rwy in Sight

Not sure what you mean by "inbalance of 500kgs".
Speaking as an ex-AF dispatcher who used to regularly dispatch the A321,what I do know is that it is a pain in the bum to trim.Because it is basically a stretched A320,and the stretch was done forward of the wings,the extra pax weight in the front makes it very forward-heavy....our standard trim for this a/c was all bags in rear hold and block some seats out occasionally and this was usually enough to keep it in trim!
Hope this helps,
777.

Every aircraft has a certain acceptable LMC (last minute change) on weight above which you need a new load sheet. No LMC is allowed for any change in fuel. You have to be careful about this on all airplanes