Only validating with jquery is not good - you leave your site open to all sorts of attacks. You must validate server-side.
I'm not a jquery user, but I imagine it may be looking for well-formed json object to be returned from your PHP script. If your PHP script is echoing errors, the jquery cannot resolve that to an object and, thefore, is probably throwing its own complaint.
So, best to resolve your PHP errors. The PHP errors are usually quite specific and even provide line numbers where the error occurred. Maybe copy/paste those errors into your preferred search engine and/or this forum to see how to resolve those issues.
Hope that helps.