I checked a few files and notice if the file still have number in the count/limit then it's alive, if it's empty then it's death.
Also if I intended put wrong password in alive file I got different notice than the death file.
For example:
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This file is alive and it show count/limit: 300 / 無制限
If put wrong password the notice is: mugeryona2291.zip ダウンロードパスが一致しませんでした (google translate mugeryona2291.zip download pass did not match)
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This file (probably) is death and it show show count/limit: - / -
If try put any password the notice is: 2290.zip----src/2290.zip_/2290.zip ファイルが存在しません (googletranslate 2290.zip----src/2290.zip_/2290.zip file does not exist)
All file older than mugeryona2290 seem death. So maybe the site eventually remove their old files after sometime.
This is exactly the same error message I get when trying different password. Good to know that the file is dead rather then cracking your head for figuring out the pass lol