Further investigation: depending on the selected archive task page, different tasks are shown. But still not related to the ready theme.

Auto comment: topic has been translated by KhB (original revision, translated revision, compare)

The same for word "class":

Auto comment: topic has been updated by KhB (previous revision, new revision, compare).

Автокомментарий: текст был обновлен пользователем KhB (предыдущая версия, новая версия, сравнить).

Try this payload on handle search:

<script\x20type="text/javascript">javascript:alert(1); ,

it should give "no such user" while it gives an error page. Might be vulnerable, I am not a tester. I was afraid to ask this in a post seeing a lot of downvotes in one of my posts.

Also words "remove", "click", "response", "function", "alert", "type", "toggle" and "codeforces" gives same result. Even parts of the link "sta.codeforces.com/s/" works in the same way.

It is really strange, that it works only on problemset page and gives only the first problem.