This top-level domain name, but this second-level domain name is not defined, will prompt that the certificate is not the trust certificate of this domain name.

Reason 4: The user does not support the SNI protocol.

This will happen in general Windows XP systems, and versions below Android 4.2 will also appear. Most of the reasons are that these systems are too old, and the number of people currently using them is very small, so it is not recommended for everyone to use them. Most of these older systems don't support the SNI(Server Name Indication) protocol, but at present, the mainstream operating systems all support this protocol, so you don't have to worry too much.