I've discovered that WPTouch assumes key files are in the site root (rather than the WP root as it should), so some things don't work when you have your WP in a subdirectory. For example, it looks for root/wp-login.php when logging in, so if (as in my case) it's actually at root/subdirectory/wp-login.php, I get a 404 error when I try to log in.
I'm wondering if this might be the cause of the "too short" commenting error that I reported long ago, which was supposed to be fixed 220.127.116.11 (but still isn't fixed for me as well as several other people, judging from the many posts on this issue here). I cannot find a plug-in conflict, and I am using the latest version. This error also does NOT occur in WPTouch 3, so you got commenting working properly in that version.