I understand this is a Debian problem, not a TDE problem, however:

On one machine I have installed Debian Stretch + TDE; no problem.
On the same machine I installed (on another partition) Buster + TDE. Seems to 
work, however most commands don't work from konsole with su.

The reason is that the path does not contain important directories.

Now the following I don't really understand:

The path is ok in the different files I could check.
I've added "export 
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin" 
to /root/.profile  but it does not work.

The result is that a command such as dpkg -i or dpkg-reconfigure does not 
work, until I issue the export PATH=  directly  in konsole

Debian is fresh installed with netinstall. This is really inconvenient, so of 
there is no way to cure it I'm going back to Stretch.

Thierry