/log @window -f "file" did work in 6.2 but in 6.3 it doesn't work anymore. Of course it's not a big issue, as without quotes it's working fine, but I didn't know what was wrong when i got this error since most commands having filenames as parameter accept/need quotes.

Perhaps you like to fix this for others (i already changed my script)

The convention that seems to exist in mIRC is that filenames should only take quotes if they can be used as something besides the last argument. In the case of /log, the filename will always be the last argument (if it's provided at all), so there's no quotes required.

I wouldn't call this a bug because it's consistent. it's just a change.. and change is inevitable.

It seems like it's a bug. Obviously because of mIRC's syntax there are some commands that require the quotes, but any command/identifier that takes a pathname should accept quotes even if it doesn't require them.

Thanks this has been fixed for the next version.