A few other suggestions:
- Try /!join #channel
- Try reinstalling mIRC to a fresh new directory and see if happens there too.
- Try another network - for example, irc.undernet.org, and see if it happens there.
This must be an issue on your side - if there was an issue with 6.16 that prevented people joining channels we'd have seen a thread before now reporting it. I would say it is either a faulty alias (which is why /remote off would not detect it, but /!join would) or a problem with the server(s) you are using.
If /!join works by the way, type /alias /join to delete whatever alias you have causing issues with /join.
Regards,