With

sub HTML_VALIDATOR = { 'http://html5.valdiator.nu/?out=json }

in lib/DBDefs.pm (or another working validator.nu instance)

Running

SAVE_HTML=1 carton exec -Ilib -- prove -v t/tests.t :: --tests 'MusicBrainz::Server::Controller::Artist::Edit$'

in my MusicBrainz VM (and also on rika) I get

unexpected end of string while parsing JSON string,
at character offset 24576 (before "(end of string)") at lib/MusicBrainz/Server/Test/HTML5.pm line 215.

So that is at decode_json

Using save_html to also save the json we get from http://html5.validator.nu/?out=json
I can see that the file is only one line (normal for json) and it is indeed 24576 characters (a bit more bytes) long and stopping in the middle of a string.

Using curl to post the html file I also saved works:

curl -X POST -d @error115.html http://html5.validator.nu/?out=json --header "Content-Type: text/html"

That gives a complete and valid json file (32 kB).

EDIT:
I forgot something important: On jenkins (ci.musicbrainz.org) HTML_VALIDATOR is set to "" (disabled), so this test is skipped. Otherwise this would possibly also fail on jenkins.