The message "Server error" means that the remote server returned an HTTP 500 Internal Server Error response. This means that the server encountered some sort of internal problem and failed to handle the request.
You can retry, but if that doesn't work, then you need to contact the party responsible for maintaining the server and ask them to look into it. The server probably has logs that can be used to determine what the problem is. However, there's nothing you can do, since this is a server error, not a client error.
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