I manage to recover the cois using a 20 september backup
but still how was that possible? the coins just disappear?
coins never disappear
u dont have coin in ur wallet
the coins are in the blockchain
they get never lost
when u dont see ur coins u run the repairwallet command in console
if u still dont see ur coins u use a backup of wallet.dat
and resync from a sure clean blockchain backup
so the answer to ur question:
ur wallet which enable u to access the coins in blockchain
was in a not proper state
such things can happen and as long as u have wallet.dat
backups there will be a way to fix it
but coins dont disappear