On tl-4.12.0 server, when trying to terminate a user session with a username containing non-ASCII characters the session is not terminated. You get a messages that all sessions are terminated already and nothing happens to the session.
Tested on nightly build 6679, with system locale set to 'C.UTF-8' on Ubuntu20.04. It now works fine to terminate a session for user named 'testör' via tlwebadm.
I could reproduce this on SLES 12 with ThinLinc 4.12.0. Fixed when upgrading to current build.
Terminating a session for a strictly ASCII user worked both in the old and new version.