Torrent + Samba copy = freeze

Hello,

I need your help. I've been struggling for more than a week with a problem.
I tried many many things to solve it, but none of them helped.

I have an RT-AC56U router with original latest firmware (I tried your firmware as well).

When I torrent some files with Download Master or Transmissions and meanwhile I copy large files from SMB share of the connected USB 3 HDD (ext4) after some minutes the router freezes and needs to turn it off and on.

I tried to lower the number of max. peers, add swap file, magic variables before starting transmission, check the SoC temperature, check log files, different filesystems, etc.

Is it a known issue?
Please help me.
Thanks.