Also getting this issue when trying to approve a cloned email. I have used the Email Actions > HTML Tools > Replace HTML function with all of these to update the source code and create my emails. This just started happening in the last 2 hours as I was able to approve the same type of emails earlier. I also tried creating a new email and just added a few lines of regular text using the WYSIWYG editor in the body but I was able to approve that.
Edit: I'm also doing this in Design Studio