MSSQL17 wasn't down. I believe you must be wrongly using the PROXY service only for OUTSIDE access for internal purposes, this outside proxy is not supposed to be used for anything but temporary management access to the servers.
It's not designed to be used for internal, and sometimes during maintenance periods it will go down unannounced, since it is a non critical service, and actually keeping it down sometimes prevents abuses of people using it internally, or from outside with unaccounted bandwidth utilization.
By using the external proxy internally, your making your website substantially slower, giving it more points of failure, and bottlenecking your own app.