I was using the non-V10 web client and have been having the "Couldn't save this note" issue since early this week.
Evernote has become Nevernote sighhh.
I found a way to reproduce in non-V10:
Create a new note.
Type some text. Note saves.
Type some underlined text. Note saves.
Press enter to make a new line. Note does not save and the error is shown.
After much frustration and not knowing about the V10 interface, having enabled that the problem seems to have gone away for me.