When I click "count triples in all graphs", the UI doesn't change and I see the following in the Chrome console:
I've experienced this in Fuseki 2.6.0 when using the WAR too. I'm also using the "tdb:unionDefaultGraph true" configuration setting.
I've used someone else's install of Fuseki 2.5.0 and it works for them. I don't think they're using "tdb:unionDefaultGraph true", and I'm not sure if they're using the WAR or a standalone. Since there is no Server: Apache-Coyote response header, I'm guessing they're using the standalone. I don't know if that would make a difference.