Over the past few weeks I’ve run into the same problem time and time again – trying to make AJAX calls with jQuery 1.5+ and receiving an
invalid label error message every time. I typically use JSON as well, and it would always break.
I couldn’t figure it out until I stumbled on a recent Stack Overflow post with the answer. The error was being caused by a de facto plugin that I use, Jörn Zaefferer’s Validation plugin. Apparently it doesn’t play nice with jQuery 1.5.
To fix, disable the plugin or, more ideally, use this patch on Github.