I’ve found that pasting into notes fields should not contain special characters, which are often found when copying/pasting directly from web pages. Introducing these characters often will result in RM error messages like you noted.
To get around this by copying from web site, pasting into Notepad (on Windows) or a plain text file in TextEdit (on Mac), doing any needed formatting, then pasting it into the notes field.
If you have a notes field that already has out of bounds characters in it you can open the field, get past the various error screens, cut the entire contents / past into Notepad or TextEdit, then copy/pasting back into the notes field.
For sure, I consider this a bug, but since it’s been around forever so a workaround is better than nothing.