You do not see the above WAL file in the archive directory ? "/mnt/server/archivedir" is shared between master and slave databases ? The file may have been removed from the archive location.
Strange !! Yes, this specified folder file "/ mnt / server / ArchiveDir". At this shared folder between master and slave.
I suspect that archive cleanup job is cleaning up the archives. "archive_cleanup_command" removes any archives not needed for recovery.
Since the the archive destination is shared across master and slave. Archives are removed and Standby is not getting the required archives.
To avoid this, before you do any tests, i suggest remove "archive_cleanup_command" parameter and see if you are still facing the same problem.
