The current configuration of this forum has a problems with titles of threads that contains one or more slashes (or backslashes).
It is directly connected with generation of non-standart thread link.
In most cases slash in URL mean division by subdirectories.
If we click on that link (e.g.: http://masterrussian.net/f39/russian...ack-18133-new/ , it's URL for thread `Russian\English and back.') then web-server is can't find the directory `russian' inside `f39', and even more so it's can't find the directory `english-back-18133-new' inside `russian' (which doesn't exist). And browser shows up 404 error.