Yeah, same problem here. In my case I installed Kibana and wanted authentication on my ES server because of that, and then nextcloud fulltext search stopped working 
It looks like some stuff has changed with the libs fulltextsearch_elasticsearch uses: You are not supposed to enter your username and password in the host field but instead use a second argument for this.
Changed the following:
line 385 of fulltextsearch_elasticsearch/lib/Platform/ElasticSearchPlatform.php no reads:
$cb = ClientBuilder::create()
->setHosts($hosts)
->setBasicAuthentication('username', 'password')
->setRetries(3);
and not:
$cb = ClientBuilder::create()
->setHosts($hosts)
->setRetries(3);
Ofcourse this is just some ducktape, but for now the module works again 
I think the best fix here would be to add 2 fields to the config form and use them like I did above. THe suggestion to use your creds in the url like they do in the example just doesn’t work anymore. BUt those are just my 2 cents, this is not my project and I’m not a nextcloud developer 