As said here: https://www.concrete5.org/index.php?cID=750921 when I try and save any text with redactor, I get the index page's mark-up as the error message.
Someone suggested that this could be an ajax error however I really have no idea. There is no error log produced and I am at a loss as to what is actually causing this to happen.
The strange thing is I can leave the content block empty and it will save fine but when I type anything into the box and hit save it throws an error.
The site has been working for the last 3 months without issue so I'm not sure why it has suddenly stopped working.
If anyone could help, this problem is driving me and my client nuts.
Edit: On further testing this problem is localized to the server which is running the website because it works absolutely fine on my local version of the website. It also seems to not only affect redactor but also any form.
Updating to 5.7.4.2 solved my problem.
Thanks to @pc-shooter for the suggestion!