There is a "Connection Issues" section in this forum in which the very first post at the top is regarding this very error message:
https://forums.mirc.com/ubbthreads.php?ubb=showflat&Number=37425&page=1#Post37425In short, it's an extremely generic error for what could be any number of issues with your connection, hardware, or software. There's no simple solution that works for everybody, it depends on what your issue actually is.
