It appears nexmo recently changed their server, but are still keeping the old one to redirect to the new one, before they eventually take it down. “/tool fetch” doesn’t cope well with redirects. Clearing the cache should point it to their new server.
Did not help much, for now I resorted to PHP nexmo library that’s running on my server, and fetch is working normally there, maybe at some point add lighttpd server on Metarouter, and put scripts there.