I was also getting this error also, but I have found two solutions that work.
1st — easiest way: When you compose a message, in the bottom right of the screen is a toggle triangle. Click on that, and one of the options is to use Plain Text. This completely eliminates the problem, and it stays on plain text, from one session to the next, so it's the better solution.
2nd way — a little trickier: When composing a message, select all the text in the message. Then in the bottom left of the compose window, right next to the Send button, is a button that is an A with an underline. Click on that, and a toolbar pops up. At the very right of this toolbar is a button that looks like a Tx. If you hover over it, it says "Remove Formatting." Click on that button, and the problem is solved, until the next time you compose a message.