Are you copying and pasting the response from another application? if so, there are possibly characters you can not see called escape characters. Programmers know them for example as a carriage return = /r/n, but in windows it may be /n only. So, if I were you, try responding to the inquiry by typing in the response manually and not copy/paste and be sure to not put too many line breaks (Return key).
I originally typed it right in the form. There was an exclamation point in it. Then it disappeared and I had to type it again, that time leaving out the exclamation point. Still no go. I'll try again from scratch by clicking on the respond button.
Tried two more times, no luck. I started from the email notification each time and clicked on the Respond button. Have you tried to respond to a review lately? Could you try it and see if yours goes through?
Are you writing a long response? If so, you could be over their max even though it doesn't appear to be. The reason I say that is that some carriage returns (newline or return key) input a /n and some input a /r/n. You can not see these characters but they are there. If you are right on the limit and you have /r/n carriage returns, it may kick you back. This is just a guess.