That is correct, the warning is displayed when the file is unreadable for any reason or contains some Unicode characters in name. One can check which files caused warnings in logs: C:\ProgramData\Veeam\Backup\tape_job_name
Also I can explain my question and the warning when system files are backed by file to tape job in 64-bit Windows. The process that collects the hierarchy is a 32-bit process (Veeam Installer Service) meanwhile the Veeam data mover which reads and transfers the files is a 64-bit process. When a 32-bit process tries to read files in C:\Windows\System32 directory, Windows redirects
it to to C:\Windows\SysWOW64 where the files are actually located so Installer Service 'thinks' the files are there. But a 64-bit process cannot find these files in C:\Windows\System32 directory and is not redirected, as a result you receive a warning.