Running into a problem configuring S/MIME on one PC. It works on a second PC. Both are Win 10 Enterprise, both using Microsoft Outlook 365, both have the same cert installed and (according to Sectigo) installed correctly. Appears to be a Windows/Office issue. On the "bad PC", the only difference I can find is that, when selecting the certificate using File > Options > Trust Center > Trust Center Settings > Email Security > Encrypted email > Settings > Choose, the hash algorithm and encryption algorithm are grayed out.
Trying to send signed email on the bad PC, Outlook pops up an error "The operation failed. The messaging interfaces have returned an unknown error. If the problem persists, restart Outlook." Which I've done. And re-installed Outlook, too. Re-installing Windows also didn't work.
Found the below link as well but there's no resolution te.