We recently added a permission that can be used to restrict this, but it should be defaulted to on (unless your admin turned it off). Can you check to make sure you have permission? Also, if it changed in an unexpected way, can you reach out to support so we can look into it?
I am experiencing something similar but with uploading code into the editor. We have a weekly newsletter and this has never been a problem for the past 2 years. but today we see this message. not sure what we do to troubleshoot. any guidance is appreciated.