For the past week or so my team have been randomly getting the above message when trying to edit content.
I found this post related to the message:
However, they are seeing it when just doing simple things like editing content in a text box. There are no sensitive scripts or iframes involved.
The site has been running for 18 months and we have had no previous issues like this.
As mentioned on the above post, I am able edit the content fine as as super-admin on a multisite installation.