I use Linux too and the same thing happened to me. A first for me on Linux to have a popup like this. I was about to call the Linux virus police lol. These sorts of things are unheard off.
Like Bozza said, it is probably something to do with the ads. I tried this in FF, Chrome, Opera and it happens across all web browsers.
In Safari on iPad gen 1, it opens up a new tab with the Isuebackupsite message above. Just have to close the tab. In Chrome I think it's also got into my bookmarks. Ran malware last night so I'll see if it found anything.