Hello, custom scripts are usually responsible for such an issue, try typing "/remote off" in mIRC to disable all the scripts, then see if it's working. In any case you can turn back on your scripts with "/remote on".
If it's working with /remote off, it means that a script is responsible for the issue as mentioned, and there one way to find which one is to unload your scripts files one by one and testing each time if messages are working ok.