Hello,
Does anyone know why the gas price has just multiplied by a factor 5 in only two days when you buy BNT with ETH ? Two days ago, it cost me 5 euros and now everyone is paying at least 25 euros (cf bancor smartcontract transactions fees on etherscan)... It is really painful just for a little transaction

Current gas prices are high. You should check
https://www.ethgasstation.info/ to get more information on the current gas price. The SafeLow is 21 at the moment, which is on the higher side.