Error on attempt to download a file with language specific symbols in name (e.g. Tést.jpg):
Internal error: Downloading the file failed. Message Downloading the file failed. File FileManager.php Line 955 Type PleskUtilException
/usr/local/psa/admin/logs/panel/logwith enabled debug the error message is:
failed to open stream: FTP server reports 550 The system cannot find the file specified
The behavior was considered as internal software issue with #PPA-2087 and it was fixed in PA 11.5 MU#11. File manager can't download files with non-ASCII characters in path. FTP protocol doesn't support UTF-8 names generally.
In order to fix the issue please install update 11 as described in the article.