I try to change my avatar to use gravatar, and when I clicked on Test in box, an error came:I don't think an add-on is related here.
It's the second error in this post:
The specific code causing the exception to be logged is our own code. It's worth noting we are specifically catching the exception here and merely logging it for your info. This is not interrupting users from registering. With that in mind it would actually be safe to ignore the error.
That said, the error itself is indicating a problem reading the response from Gravatar. This isn't something we directly have control over. There's possibly a network related cause (such as a firewall or other connection issue) or problems with the response being received from Gravatar.
It would be worth checking with your host in the first instance.
If you don't use Gravatar, you could simply disable it.