I've made repo with test:
https://github.com/gimer/curve25519-sign-testI've also added class files to the repo, so it's enough to replace "nxt-0.7.2\webapps\root\WEB-INF\classes\nxt\crypto\Curve25519.class" with file from the repo and it should work.
I hope that u won't create a "solution" that
leaks private key bits. I'm against the changes until I see a formal proof, so u better talk to Jean-Luc.
I don't want to be offensive, but it seems that the curve25519.java file is a black box for you. Talking about leaking private key bits from those changes is ridiculous.
But if BCNext, Jean-Luc and you refuse to make changes to that file then we just have to live with a (in my opinion annoying) every now and then failing signature/verification system.