I've seen this happen with 6.12 as well, so it seems to be an issue with the 6.1x series, however, mine only occurs after I've copied a script into the mIRC directory, and haven't actually run the mIRC.exe before doing so.
The simple solution for me is to make a logs directory in my script.
However, I personally think that the setup program for mIRC should create all the folders instead of relying on mIRC.exe to create them.