It happens to me whenever I try quoting users with non-ASCII characters in their username. Clicking on "Toggle BB Code" (see the image below) and removing the offending characters from the username allows me to quote them. I never had this happen when I posted on Kiwi Farms, so I'm not sure why it's an issue here.
View attachment 318
While I'm on the subject, I also get the error message if I insert an emoji with the emoji picker/quote a message that contains emoji.