By surfing the forums I found that a lot of users have problems validating their ssml. Constant issues are misplaced tags, unclosed tags, missing brackets or extra spaces inside of the tags.
Because error message says only, that input was invalid users don't know what was invalid and they keep asking the same thing - to explain them what is wrong with their ssml, which most of the time is one of the problems I mentioned before.
My suggestion would be to add a better validation for ssml, including 'Voice & Tone' tab. It should give better messages to at least help users to pinpoint what could be wrong, because current message is not really helpful.