Answer
Don't assume the certificate problem is on the FTP server just because that's where the file itself is stored — the actual communication chain behind a file-size/status check like this can involve other components (application server-side integration pieces) that have their own certificates, and a certificate error here can point to one of those, not the FTP server's own (nonexistent) certificate setup. In a confirmed case, the actual cause was a single expired/invalid certificate elsewhere in that communication chain — finding and updating whichever certificate had actually expired (by checking each component in the real chain rather than only the FTP server) resolved the approval failure.