transmission-qt only run on KDE Plasma

Transmission daemon 2.94-2 (like previous releases) has been working fine on Debian Buster, along with Transmission Web Interface, but not with transmission-gtk and transmission-qt clients, outside the KDE Plasma realms.

transmission-gtk only seems to run if the service is not running (something weird because it’s supposed to be a service client, not a standalone client).

Tue Dec 29 22:20:39 2020        error           Couldn't bind port 51413 on 0.0.0.0: Address already in use (Is another copy of Transmission already running?)
Tue Dec 29 22:20:39 2020        error           Couldn't bind port 51413 on ::: Address already in use (Is anothercopy of Transmission already running?)

transmission-qt doesn’t even run, it only display this error message:

org.kde.knotifications: env says KDE is running but SNI unavailable -- check KDE_FULL_SESSION and XDG_CURRENT_DESKTOP

I tried to solve it (with no success), setting those two variables in ~/.bashrc like this:

export XDG_CURRENT_DESKTOP=KDE
export KDE_FULL_SESSION=true

I tried setting them in ~/.profile but it was the same.

So, transmission-qt doesn’t run on Openbox, IceWM, LXDE, Mate or Xfce, same knotifications message on all of them.

Anyone else with the same issue? Does anyone found a solution?