WHen there is a massive ctcp version flood from a IRC botnet, mIRC tends to close the connection with [15:16] * Send error ([10053] Software caused connection abort)

There isn't really much that mIRC can do if it encounters a flood of almost any kind.

Please also note that there is a huge sticky regarding this error number, including some possible fixes, right here

Is your problem that it's closing the connection? Or that it's specifically closing it with 10053?

For the former problem, there's really no way around this. If you get flooded by a botnet and you don't have flood protection enabled, you will disconnect. One thing you can and should do is enable server-side flood protection if available (see your server info for instructions, it depends heavily on the network) or use mIRC internal flood control. Realize, though, that you can't be 100% safe from floods. It is not a bug that you disconnect from a flood.

For the latter, I would say it's not so important why 10053 is your disconnection message. In fact, to the server it's probably closer to "Max SendQ exceeded", not 10053. 10053 is just a generic error that displays when a connection is abruptly cut off without a graceful TCP connection teardown. The disconnection is expected (see previous paragraph), therefore "how" you get disconnected isn't really the issue anyway.