Answer from the support :
Dear MEGA user,
Thank you for your support and for using MEGA!
We do not recommend any third-party tools. Our onsite download is already using multichannel downloading. The Internet Download Manager, JDownloader or any other third party download client have not been approved or authorized by MEGA and should only be used at your own risk.
Also, these foreign tools cannot detect your quota and could limit your bandwidth.
To manage your transfers, the best way is to use MEGAsync - our desktop sync and download/upload client. It can be download here: https://mega.nz/sync
From MEGAsync interface, click the gear icon at the top-right corner and select “Import links” from the dropped-down list.
Maybe, as Alfred says, a temporary problem with an overload.
The first month I use, all was perfect…