I see. Sorry I was confused because it all works as expected for me.
When I go to the "Translations" tab in the Plugins directory, and I click the "Contribute translations for other languages" I am redirected to a page in AMOS with the filter pre-configured to show the strings just for that plugin. I only need to select the language I want to translate to.
Also when I follow https://lang.moodle.org/local/amos/view.php?v=3900&t=1616746636&l=cs&c=local_catdup I get the following as expected without any error:
However I was able to reproduce the problem when I used the "Log in as" feature and see it via your profile. I reported it as MDLSITE-6425.
Thanks again for raising it.