Windows 7 Backup on Synology: Smartcard Certificate Authority Untrusted

On setting up a backup for the NAS on W10 Pro 20H2, get the following COM error 0x80090352:

An untrusted certificate authority was detected While processing the smartcard certificate used for authentication. Please contact your system administrator.

The certificate on the Synology NAS is the basic one set up with LetsEncrypt: so named storage.synology.me. Presumably Windows takes a copy of this certificate and sets the untrusted flag. However, there seems nothing in the certificate that indicates it's a smartcard cert- does the W7 backup program make the conversion? There's nothing like a copy of it in the MMC > Certificates store either.

Also, when different NAS subdirectories are specified, the W7 backup program tends to want to randomly select NAS subdirectories as candidate for either a 0X80070043 (Network Name cannot be found) or the above 0x80090352. Thus it is possible to reselect a subdirectory in the Select a Network Location dialog to get a different error.

Is there something in the Windows Group Policy settings causing this, or can anything more be done on the NAS side?

As a matter of interest, this issue with the same NAS never arose on another system with W10 Home edition installed, and the newer W10 "Backup Using File History" has no issue writing to any directory on the NAS drive.

The backup proceeded once the IIS Crypto Best Practices option for TLS was applied. Please refer to the LetsEncrypt thread for details.