Uploading a JMX file may result in a "Validation failed" error without apparent reason
Besides non ascii characters (from x'80' thru x'FF'), also the presence of characters in the range x'00' through x'1F' (except x'0A' and x'0D') may cause this failure
BLAZEMETER PERFORMANCE TESTING
To determine the culprit, edit the JMX with an editor that reveals non printable characters (such as Sublime Text). Then search for characters in the problem range. You will find invalid chars such as 1E in lines like this:
Remove the invalid characters, save the file and upload it again.