Yesterday I, too, got the "Note is invalid" message when editing a note I've had for years and years. Much worse is that Evernote then proceeded to completely erase all contents from my note. That freaked me out enough that I exported HTML files for all of my notes, and after doing that I noticed that the note whose contents Evernote had erased had somehow been reverted to a version of that note from about 17 months ago(!). I'm on Evernote Version 7.0.2 (456267 App Store) and macOS High Sierra 10.13.3. DATS-INTRAS, the one step I'd advise you to take is to immediately back up your note(s) in case Evernote does to you what it did to me (i.e., wipe out important note contents).