While we have, on occasion, been able to narrow 10053 down to a specific cause for a particular user, it is a generic connection error with many possible causes.
One item you didn't mention on your list is anti-virus software. If you are using anti-virus software, you would need to check if the issue is still present after you 1) add an exception for mIRC, 2) disable the anti-virus software, 3) really disable the anti-virus software (some anti-virus software says it is disabled but continues working in the background in some way).
Some servers perform a client version check on connecting to the server, so it may be that they change how they poll an IRC client based on that. mIRC already polls the server connection to check if it has timed out (Enable "Check connection timeout" option in Options/Connect/Options dialog). If it has timed out, this will result in the 10053 error (I don't know if other IRC clients do this - if they don't, it may appear that they are still connected when they are not).
If you find a solution, please let us know.
