I found that even I selected iPhone and PSP x264 porfile in video ouput, I still get "Cannot load preset error". Attached the log file. Probably there is the infamous forward/backward slash problem?
The backslash path is for chinese localization. The rest it looks fine.
It was a change in one parameter name
Fixed in nightly (http://www.avidemux.org/nightly/win32 revision >=8194),
Make sure to delete
$HOME/pluginSettings/x264/1/*