This is likely unrelated to Tor Browser (except that Tor Browser is being more strict about TLS being properly configured on the server side, it seems...), the server itself is at fault, the administrator has failed to configure TLS correctly.
It is missing an intermediate certificate to complete the "trust" chain between it's certificate and a trusted anchor, therefor Tor Browser cannot work back to a trusted anchor to ensure that it has been issued by a CA it trusts.
COMODO has a root certificate
COMODO (or some bored student from Iran...) signs an intermediate certificate
B, this allows them to keep the root of their trust,
A, offline and safe. If
B is compromised it is easier to revoke and replace than
COMODO signs your TLS certificate,
The server, however, is only issuing
C and the browser has no indication that
B exists, it cannot find a link to
A. The server should issue both
B so your browser can find
A and know that
See the Qualys SSL Labs report for
This server's certificate chain is incomplete. Grade capped to B.