i can´t help u to fix but u can use the app image that works for me . if u want too u can find there https://mediathekview.de/download/
with the installed version i had too problems .
it has work long time at me but last when i started i have also problems . i think medithekview need an update .
This is what i get in terminal :
mediathekview
Fehler: Beim Laden der Klasse mediathek.Main ist ein LinkageError aufgetreten
java.lang.UnsupportedClassVersionError: mediathek/Main (class file version 59.65535) was compiled with
preview features that are unsupported. This version of the Java Runtime only recognizes preview features for
class file version 60.65535